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.