Fluid: restructuring and commenting.

tl;dr : making Fluid maintainable, no changes in code execution and logic.

This is a pretty extensive restructuring of the Fluid source tree.
It was neccessary because source and header files were getting
much too big to handle. Many source files had no header, and many
headers declared functions that were in diffrent source files.

Reorganized much of the include statements.
Added comments to some of the files.
Added Doxygen configuration file for standalone Fluid docs.

Tested everything by rebuilding Fluid .fl designs with the resorted
version of Fluid.

fluid/CMakeLists.txt

@@ -33,8 +33,10 @@ set (CPPFILES
   fluid.cxx
   function_panel.cxx
   pixmaps.cxx
+  shell_command.cxx
   template_panel.cxx
   undo.cxx
+  widget_browser.cxx
   widget_panel.cxx
 )
 
@@ -42,19 +44,30 @@ set (CPPFILES
 
 set (HEADERFILES
   CodeEditor.h
+  Fl_Function_Type.h
+  Fl_Group_Type.h
+  Fl_Menu_Type.h
   Fl_Type.h
   Fl_Widget_Type.h
+  Fl_Window_Type.h
   Fluid_Image.h
   Shortcut_Button.h
   StyleParse.h
   about_panel.h
+  align_widget.h
   alignment_panel.h
+  code.h
   comments.h
+  factory.h
+  file.h
+  fluid.h
   function_panel.h
   print_panel.h
   pixmaps.h
+  shell_command.h
   template_panel.h
   undo.h
+  widget_browser.h
   widget_panel.h
 )