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.
Fix usage of vsscanf in Fl_Unix_System_Driver::clocale_vsscanf().
Final commit of all patches provided by the OP of issue #937 - if
everything works now.
FL_OVERRIDE is defined as `override` for C++11 and higher
FL_OVERRIDE is defined as `override` for VisualC 2015 and newer
Don't interfere with Fl_Widget::override()
