Add Fl_Grid widget and test and demo programs

- FL/Fl_Grid.H: header file
- src/Fl_Grid.cxx: implementation

- examples/grid-simple.cxx: simple example program
- test/cube.cxx:            use Fl_Grid for layout
- test/grid_alignment.cxx:  test cell alignment and other functions
- test/grid_buttons.cxx:    demo program as discussed in fltk.general
- test/grid_login.cxx:      like test/flex_login.cxx but with Fl_Grid
- test/flex_login.cxx:      modified to match test/grid_login.cxx

4 files changed, 56 insertions, 4 deletions

diff --git a/examples/.gitignore b/examples/.gitignore
index f62a5e4db..d4a21b441 100644
--- a/examples/.gitignore
+++ b/examples/.gitignore
@@ -12,6 +12,7 @@ callbacks
 cairo-draw-x
 chart-simple
 draggable-group
+grid-simple
 howto-add_fd-and-popen
 howto-browser-with-icons
 howto-drag-and-drop
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index b1409d12e..ea0b8487a 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -1,7 +1,7 @@
 #
 # CMakeLists.txt used to build example apps by the CMake build system
 #
-# Copyright 2020-2022 by Bill Spitzak and others.
+# Copyright 2020-2023 by Bill Spitzak and others.
 #
 # This library is free software. Distribution and use rights are outlined in
 # the file "COPYING" which should have been included with this file.  If this
@@ -31,10 +31,11 @@ file (MAKE_DIRECTORY ${EXECUTABLE_OUTPUT_PATH})
 ############################################################
 
 set (SIMPLE_SOURCES
-  chart-simple
-  callbacks
   browser-simple
+  callbacks
+  chart-simple
   draggable-group
+  grid-simple
   howto-add_fd-and-popen
   howto-browser-with-icons
   howto-drag-and-drop
@@ -45,8 +46,8 @@ set (SIMPLE_SOURCES
   howto-remap-numpad-keyboard-keys
   howto-text-over-image-button
   menubar-add
-  nativefilechooser-simple-app
   nativefilechooser-simple
+  nativefilechooser-simple-app
   progress-simple
   shapedwindow
   simple-terminal
diff --git a/examples/Makefile b/examples/Makefile
index 57d87de39..43f1d3119 100644
--- a/examples/Makefile
+++ b/examples/Makefile
@@ -31,6 +31,7 @@ ALL = animgifimage$(EXEEXT) \
       callbacks$(EXEEXT) \
       chart-simple$(EXEEXT) \
       draggable-group$(EXEEXT) \
+      grid-simple$(EXEEXT) \
       howto-add_fd-and-popen$(EXEEXT) \
       howto-browser-with-icons$(EXEEXT) \
       howto-drag-and-drop$(EXEEXT) \
diff --git a/examples/grid-simple.cxx b/examples/grid-simple.cxx
new file mode 100644
index 000000000..f08a5957b
--- /dev/null
+++ b/examples/grid-simple.cxx
@@ -0,0 +1,49 @@
+//
+// Fl_Grid Example Program for the Fast Light Tool Kit (FLTK).
+//
+// Copyright 2021-2022 by Albrecht Schlosser.
+// Copyright 2022-2023 by Bill Spitzak and others.
+//
+// This library is free software. Distribution and use rights are outlined in
+// the file "COPYING" which should have been included with this file.  If this
+// file is missing or damaged, see the license at:
+//
+//     https://www.fltk.org/COPYING.php
+//
+// Please see the following page on how to report bugs and issues:
+//
+//     https://www.fltk.org/bugs.php
+//
+
+// This example program is also included in the documentation.
+// See FL/Fl_Grid.H
+
+#include <FL/Fl.H>
+#include <FL/Fl_Double_Window.H>
+#include <FL/Fl_Grid.H>
+#include <FL/Fl_Button.H>
+
+int main(int argc, char **argv) {
+  Fl_Double_Window *win = new Fl_Double_Window(320, 180, "3x3 Fl_Grid with Buttons");
+  // create the Fl_Grid container with five buttons
+  Fl_Grid *grid = new Fl_Grid(0, 0, win->w(), win->h());
+  grid->layout(3, 3, 10, 10);
+  grid->color(FL_WHITE);
+  Fl_Button *b0 = new Fl_Button(0, 0, 0, 0, "New");
+  Fl_Button *b1 = new Fl_Button(0, 0, 0, 0, "Options");
+  Fl_Button *b3 = new Fl_Button(0, 0, 0, 0, "About");
+  Fl_Button *b4 = new Fl_Button(0, 0, 0, 0, "Help");
+  Fl_Button *b6 = new Fl_Button(0, 0, 0, 0, "Quit");
+  // assign buttons to grid positions
+  grid->widget(b0, 0, 0);
+  grid->widget(b1, 0, 2);
+  grid->widget(b3, 1, 1);
+  grid->widget(b4, 2, 0);
+  grid->widget(b6, 2, 2);
+  // grid->show_grid(1);     // enable to display grid helper lines
+  win->end();
+  win->resizable(grid);
+  win->size_range(300, 100);
+  win->show(argc, argv);
+  return Fl::run();
+}