GTK 3 prep: do not include individual GTK/GDK headers.

GTK 3 prep: do not include individual GTK/GDK headers.

This is the first of several cleanup steps recommended by the GTK 2->3
migration guide.

I intend to begin work towards compatibility with GTK 3, but without
breaking GTK 2 and even GTK 1 compatibility in the process; GTK 2 is
still useful to _me_ (not least because it permits much easier support
of old-style server-side X11 fonts), and I recall hearing a rumour
that at least one kind of strange system can only run GTK 1, so for
the moment I don't intend to stop supporting either.

Including gdkkeysyms.h is not optional in GTK 2, because gdk.h does
not include it. In GTK 3 it does, so we don't explicitly reinclude it
ourselves.

We now build cleanly in GTK2 with -DGTK_DISABLE_SINGLE_INCLUDES. (But
that doesn't say much, because we did already! Apparently gdkkeysyms.h
was a special case which that #define didn't forbid.)