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.

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