Fix gdk_xid_table_lookup_for_display deprecated.
Fix gdk_xid_table_lookup_for_display deprecated.

According to the documentatiom, GTK+ only stores windows
in its X id table nowadays and the recomendation is to use
gdk_x11_window_lookup_for_display() instead of 
gdk_xid_table_lookup_for_display().

This fixes commit #620e660f89

Signed-off-by: Germán Póo-Caamaño <679831b636f7f6776b5d7665ebec4c0074d994a9@gnome.org>
GNOME/gnome-panel,GNOME/gnome-panel