This property was only used until now when there was neither an icon nor a label set, for arrow direction and popover placement.
Starting with Gtk4, a GtkMenuButton with a label shows an arrow at the right (in LTR) of the label. Allow disabling the arrow or changing its direction using the direction property, to have a way to restore a Gtk3- like look or to improve popover placement.
Fixes #2811 (closed).