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 remembering the first-leave and last-enter events 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
0dfdc9371e