Remove deprecated function Fl::set_idle()

This turned out to be more complicated than just to delete a function
because it was used internally, and the callback signatures were a bit
flaky. I also added a lot of documentation to clarify matters.

FL/Fl.H: document idle callback signatures, make some internal
  functions of class Fl private, add public Fl::idle() accessor (read-
  only), add Fl::add_idle(Fl_Old_Idle_Handler cb) to enable using
  old-style idle callbacks w/o 'data' argument.

FL/forms.H: replace Fl::set_idle() with Fl::add_idle().

src/Fl.cxx: rename private Fl::idle_ with trailing underscore.

src/Fl_System_Driver.cxx: use new public accessor Fl::idle() to access
  Fl::idle_ which is now private.

src/Fl_add_idle.cxx: improve documentation, clarify idle callback
  matching, add example code in docs, rename methods,  add overloaded
  Fl::add_idle(Fl_Old_Idle_Handler cb).

src/Fl_win32.cxx: use public Fl::idle() rather than private member.
src/drivers/Unix/Fl_Unix_System_Driver.cxx: same as above.
src/Fl_cocoa.mm: same as above.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/Fl.cxx b/src/Fl.cxx
index bae7f1328..b1c38caf6 100644
--- a/src/Fl.cxx
+++ b/src/Fl.cxx
@@ -580,7 +580,7 @@ void fl_trigger_clipboard_notify(int source) {
 ////////////////////////////////////////////////////////////////
 // idle/wait/run/check/ready:
 
-void (*Fl::idle)(); // see Fl::add_idle.cxx for the add/remove functions
+void (*Fl::idle_)(); // see Fl::add_idle.cxx for the add/remove functions
 
 /*
   Private, undocumented method to run idle callbacks.
@@ -611,9 +611,9 @@ void (*Fl::idle)(); // see Fl::add_idle.cxx for the add/remove functions
 */
 void Fl::run_idle() {
   static char in_idle;
-  if (Fl::idle && !in_idle) {
+  if (Fl::idle_ && !in_idle) {
     in_idle = 1;
-    Fl::idle();
+    Fl::idle_(); // call the idle callback stored in Fl::idle_ == Fl::idle()
     in_idle = 0;
   }
 }