GTK 3 prep: stop using *nearly* all GTK deprecated functions.

GTK 3 prep: stop using *nearly* all GTK deprecated functions.

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.