Fix hover state after DND

Fix hover state after DND

During a drag-and-drop, our pointer grab keeps enter/leave events from
being delivered. That means that after the DND ends, whatever actor is
under the pointer won't have received the enter event it should have,
and any state or hover effect dependent on that won't work right.
By paying attention to the actors we leave and enter we can figure out
what widgets we need to call st_widget_sync_hover() on after the drag.

https://bugzilla.gnome.org/show_bug.cgi?id=640974