Make global GDK libgtk_only functions more private

Make global GDK libgtk_only functions more private

The current way of exposing GDK API that should be considered internal
to GTK+ is to append a 'libgtk_only' suffix to the function name; this
is not really safe.

GLib has been using a slightly different approach: a private table of
function pointers, and a macro that allows accessing the desired symbol
inside that vtable.

We can copy the approach, and deprecate the 'libgtk_only' symbols in
lieu of outright removal.

https://bugzilla.gnome.org/show_bug.cgi?id=739781