GtkMenu: Use gdk_window_move_to_rect() even more often

The fix in https://bugzilla.gnome.org/show_bug.cgi?id=772922 made
the positioning code use gdk_window_move_to_rect() under certain conditions.
This change removes one of the limitations: the new code will be used
even if a custom positioning function is provided.

This fixes context menus on Wayland in Firefox, Epiphany, and LibreOffice.

Fixes #1154
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00:10:57