Bug 1108 - Enter/Leave events logics wrt. skipped motion events

Bug 1108 - Enter/Leave events logics wrt. skipped motion events
Handle dropped motion events when computing crossing events,
based on a patch from Gwenole Beauchesne.
* clutter/clutter-main.c: (clutter_event_get_device): internal static
utility function.
(clutter_do_event): generate enter/leave events for all pointer
events.
(generate_enter_leave_events): modified enter/leave events generator
to work for all pointer event types. Enter/leave events are now
delivered before the motion/button event that caused the crossing to
happen.
* clutter/clutter-event.c: (clutter_event_copy), (clutter_event_free):
removed reference counting logic that is not needed when the crossing
events are directly delivered.