Make GtkMenuButton::direction more useful.

Merged Arnaud B. requested to merge arnaudb/menubutton-direction into master

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).

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