* clutter.symbols: Add clutter_actor_set_clipu() and
clutter_actor_get_clipu()
* clutter/clutter-actor.h:
* clutter/clutter-actor.c:
(clutter_actor_set_clipu), (clutter_actor_get_clipu): Add the
new units-based versions of the clip accessors
(clutter_actor_set_clip), (clutter_actor_get_clip): Reimplement
the pixels-based clip accessors as proxies for the units-based
ones