GtkMenu: Try using gdk_window_move_to_rect() even more often

Applications like Firefox (Nightly), Epiphany, and LibreOffice show context menus in the wrong locations when running on Wayland with multiple monitors. (See examples here; I originally thought this was a Mutter bug.)

This fixes the menu positioning by using gdk_window_move_to_rect() even when a custom positioning function is provided.