This is the API used by GtkMenu to properly position menus on the screen
without requiring GTK to query the menu window's position or the work
area of where the window is positioned. It makes it possible to position
popup windows properly when using Wayland.
Make this API available to external users so custom popup windows can be
positioned properly as well.
Closes: https://gitlab.gnome.org/GNOME/gtk/issues/997
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
