When an InputDevice leaves a stage we set the stage member of
InputDevice to NULL. We should also unset the cursor_actor (as the
device is obviously not on an actor any more).
When the device re-enters the Stage the ENTER/LEAVE event generation
machinery will then be able to emit the ENTER event on the Stage.
The previous state for the device is used by the click count machinery
and we should not be overwriting it at every event; instead, we should
use a parallel storage for the current state coming from the windowing
system.
The LEAVE/ENTER event pairs should be queued during the InputDevice
update process, when we change the actor under the device pointer.
This commit cleans up the event emission code inside clutter-main.c
and the logic of the event processing.
The InputDevice objects stores pointer coordinates, state, stage and
the actor under the cursor, so if the current backend provides us with
one attached to the Event structure then we want the InputDevice itself
to update its state and give us the ClutterActor underneath the
pointer's cursor.
