Instead of delegating on the parent shell of a menu item/shell on a variety
of situations, Simplify event handling so:
1) Menu item selection is handled entirely on GtkMenuItem through crossing
   events.
2) The deepmost menu shell handles clicks inside and outside of it.

This avoids the rather hard to follow gtk_widget_event() calls going on all
throughout the handling of crossing and button events, and makes menus work
again.