GtkMenu: Use gdk_window_move_to_rect() even more often

The fix in 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
1 job for fix_menu_position in 10 minutes and 57 seconds (queued for 2 seconds)
Status Name Job ID Coverage
passed build #49636