Support mouse buttons 4 + 5 (aka "side buttons") (#1076, #1068)

This work is based on PR 1068 (patch by @CendioHalim) and
extended to store button status (4,5) in Fl::event_state() like
it's done for other mouse buttons (1-3).

Changes:
- new symbol: FL_BUTTON4 = side button 1 = "back"
- new symbol: FL_BUTTON5 = side button 2 = "forward"
- modified  : FL_BUTTONS now includes bits for two side buttons

Note: the status of these new buttons is not maintained by X11,
  therefore we need to maintain them in internal variables for
  this platform.

2 files changed, 67 insertions, 38 deletions

diff --git a/FL/Enumerations.H b/FL/Enumerations.H
index 1beb450e5..97b4c7029 100644
--- a/FL/Enumerations.H
+++ b/FL/Enumerations.H
@@ -548,9 +548,11 @@ enum Fl_Callback_Reason {
 
 /**@{*/
 
-#define FL_LEFT_MOUSE   1       ///< The left mouse button
-#define FL_MIDDLE_MOUSE 2       ///< The middle mouse button
-#define FL_RIGHT_MOUSE  3       ///< The right mouse button
+#define FL_LEFT_MOUSE     1       ///< The left mouse button
+#define FL_MIDDLE_MOUSE   2       ///< The middle mouse button
+#define FL_RIGHT_MOUSE    3       ///< The right mouse button
+#define FL_BACK_MOUSE     4       ///< The back mouse button (side button 1)
+#define FL_FORWARD_MOUSE  5       ///< The forward mouse button (side button 2)
 
 /**@}*/          // group: Mouse Buttons
 
@@ -575,11 +577,17 @@ enum Fl_Callback_Reason {
                                         // correct for XFree86
 #define FL_SCROLL_LOCK  0x00800000      ///< The scroll lock is on
                                         // correct for XFree86
-#define FL_BUTTON1      0x01000000      ///< Mouse button 1 is pushed (L)
-#define FL_BUTTON2      0x02000000      ///< Mouse button 2 is pushed (M)
-#define FL_BUTTON3      0x04000000      ///< Mouse button 3 is pushed (R)
-#define FL_BUTTONS      0x07000000      ///< Any mouse button (1-3) is pushed
-#define FL_BUTTON(n)   (0x00800000<<(n)) ///< Mouse button n (n > 0) is pushed
+// Mouse buttons
+
+#define FL_BUTTON1      0x01000000        ///< Mouse button 1 is pushed (L)
+#define FL_BUTTON2      0x02000000        ///< Mouse button 2 is pushed (M)
+#define FL_BUTTON3      0x04000000        ///< Mouse button 3 is pushed (R)
+#define FL_BUTTON4      0x08000000        ///< Mouse button 4 is pushed (BACK)
+#define FL_BUTTON5      0x10000000        ///< Mouse button 5 is pushed (FORWARD)
+#define FL_BUTTONS      0x1f000000        ///< Bitmask: any mouse button (1-5) is pushed
+
+#define FL_BUTTON(n)   (0x00800000<<(n))  ///< Mouse button n (n = 1..5) is pushed,
+                                          ///< *undefined* if n outside 1..5
 
 #define FL_KEY_MASK     0x0000ffff      ///< All keys are 16 bit for now
                                         //   FIXME: Unicode needs 21 bits!
diff --git a/FL/Fl.H b/FL/Fl.H
index 57be52e5d..fceafc4c8 100644
--- a/FL/Fl.H
+++ b/FL/Fl.H
@@ -705,40 +705,51 @@ public:
     This returns garbage if the most recent event was not a FL_PUSH or FL_RELEASE event.
     \retval FL_LEFT_MOUSE
     \retval FL_MIDDLE_MOUSE
-    \retval FL_RIGHT_MOUSE.
-    \see Fl::event_buttons()
+    \retval FL_RIGHT_MOUSE
+    \retval FL_BACK_MOUSE
+    \retval FL_FORWARD_MOUSE.
+    \see Fl::event_buttons(), Fl::event_state()
   */
-  static int event_button()     {return e_keysym-FL_Button;}
+  static int event_button() { return e_keysym - FL_Button; }
   /**
     Returns the keyboard and mouse button states of the last event.
 
     This is a bitfield of what shift states were on and what mouse buttons
     were held down during the most recent event.
 
-    The legal event state bits are:
-
-    - FL_SHIFT
-    - FL_CAPS_LOCK
-    - FL_CTRL
-    - FL_ALT
-    - FL_NUM_LOCK
-    - FL_META
-    - FL_SCROLL_LOCK
-    - FL_BUTTON1
-    - FL_BUTTON2
-    - FL_BUTTON3
-
-   \note FLTK platforms differ in what Fl::event_state() returns when it is called while a modifier key
-   is being pressed or released.
-   Under X11 and Wayland, Fl::event_state() indicates the state of the modifier keys just \b prior to the event.
-   Thus, during the FL_KEYDOWN event generated when pressing the shift key, for example, the FL_SHIFT bit of event_state()
-   is 0 and becomes 1 only at the next event (which can be another FL_KEYDOWN, FL_DRAG or FL_KEYUP).
-   Under other platforms, the reported state of modifier keys includes that of the key being pressed or released.
-   Notice that Fl::event_state() returns the same value under all platforms when it's called while a non-modifier key
-   (e.g., a letter, a function key) is being pressed or released.
-   X servers do not agree on shift states, and FL_NUM_LOCK, FL_META, and
-   FL_SCROLL_LOCK may not work. The values were selected to match the
-   XFree86 server on Linux.
+    \note FLTK platforms differ in what Fl::event_state() returns when it is called
+      while a modifier key or mouse button is being pressed or released.
+
+  - Under X11 and Wayland, Fl::event_state() indicates the state of the modifier keys and
+    mouse buttons just \b prior to the event. Thus, during the \c FL_KEYDOWN event generated
+    when pressing the shift key, for example, the \c FL_SHIFT bit of event_state() is 0 and
+    becomes 1 only at the next event which can be any other event, including e.g. \c FL_MOVE.
+  - Under other platforms the reported state of modifier keys or mouse buttons includes that
+    of the key or button being pressed or released.
+  - Fl::event_state() returns the same value under all platforms when it's called while a
+    non-modifier key (e.g. a letter or function key) is being pressed or released.
+  - X servers do not agree on shift states, and \c FL_NUM_LOCK, \c FL_META, and \c FL_SCROLL_LOCK
+    may not work.
+  - The values were selected to match the XFree86 server on Linux.
+
+  \note This inconsistency \b may be fixed (on X11 and Wayland) in a later release.
+
+  The legal event state bits are:
+
+    | Device   | State Bit      | Key or Button           | Since |
+    |----------|----------------|-------------------------|-------|
+    | Keyboard | FL_SHIFT       | Shift                   |       |
+    | Keyboard | FL_CAPS_LOCK   | Caps Lock               |       |
+    | Keyboard | FL_CTRL        | Ctrl                    |       |
+    | Keyboard | FL_ALT         | Alt                     |       |
+    | Keyboard | FL_NUM_LOCK    | Num Lock                |       |
+    | Keyboard | FL_META        | Meta, e.g. "Windows"    |       |
+    | Keyboard | FL_SCROLL_LOCK | Scroll Lock             |       |
+    | Mouse    | FL_BUTTON1     | left button             |       |
+    | Mouse    | FL_BUTTON2     | middle button           |       |
+    | Mouse    | FL_BUTTON3     | right button            |       |
+    | Mouse    | FL_BUTTON4     | side button 1 (back)    | 1.4.0 |
+    | Mouse    | FL_BUTTON5     | side button 2 (forward) | 1.4.0 |
   */
   static int event_state()      {return e_state;}
 
@@ -1267,7 +1278,7 @@ public:
     time of the event. During an FL_RELEASE event, the state
     of the released button will be 0. To find out, which button
     caused an FL_RELEASE event, you can use Fl::event_button() instead.
-    \return a bit mask value like { [FL_BUTTON1] | [FL_BUTTON2] | [FL_BUTTON3] }
+    \return a bit mask value like { [FL_BUTTON1] | [FL_BUTTON2] | ... | [FL_BUTTON5] }
   */
   static int event_buttons() {return e_state & FL_BUTTONS;}
   /**
@@ -1276,15 +1287,25 @@ public:
   */
   static int event_button1() {return e_state & FL_BUTTON1;}
   /**
-    Returns non-zero if button 2 is currently held down.
+    Returns non-zero if mouse button 2 is currently held down.
     For more details, see Fl::event_buttons().
   */
   static int event_button2() {return e_state & FL_BUTTON2;}
   /**
-    Returns non-zero if button 3 is currently held down.
+    Returns non-zero if mouse button 3 is currently held down.
     For more details, see Fl::event_buttons().
   */
   static int event_button3() {return e_state & FL_BUTTON3;}
+  /**
+    Returns non-zero if mouse button 4 is currently held down.
+    For more details, see Fl::event_buttons().
+  */
+  static int event_button4() {return e_state & FL_BUTTON4;}
+  /**
+    Returns non-zero if mouse button 5 is currently held down.
+    For more details, see Fl::event_buttons().
+  */
+  static int event_button5() {return e_state & FL_BUTTON5;}
   /**   @} */
 
   /**