Building with -DGTK_DISABLE_DEPRECATED, we now suffer only one compile
failure, for the use of gtk_quit_add() in idle_toplevel_callback_func.
That function is apparently removed with no replacement in GTK 3, so
I'll need to find a completely different approach to getting toplevel
callbacks to run only in the outermost instance of gtk_main().
Also, this change doesn't do anything about the use of *GDK*
deprecated functions, because those include the entire family of
old-style drawing functions - i.e. the only way to build cleanly with
-DGDK_DISABLE_DEPRECATED will be to switch to Cairo drawing.
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