Big rework of the actor management semantics: now ClutterActor
	objects behave like GtkObjects - that is they have an initial
	"floating" reference that gets "sunk" when they are added to
	a ClutterGroup.  This makes a group responsible of de-allocating
	each actor inside it, so you just have to destroy the group to
	get every child actor destroyed.  Also, now you can do:

	  clutter_group_add (group, clutter_video_texture_new ());
	
	without having to care about reference counting and explicit
	unreffing.

	* clutter/clutter-private.h: Add private flags setter and
	getter macros.

	* clutter/clutter-actor.h:
	* clutter/clutter-actor.c: Clean up; inherit from GInitiallyUnowned;
	add a "visible" property; add the "destroy", "show" and "hide"
	signals to ClutterActorClass.

	(clutter_actor_show), (clutter_actor_hide): Refactor a bit; emit
	the "show" and "hide" signals.

	(clutter_actor_set_property), (clutter_actor_get_property),
	(clutter_actor_class_init): Implement the "visible" property; add
	signals.

	(clutter_actor_finalize): Do not leak the actor's name, if it is
	set.
	
	(clutter_actor_dispose): Emit the "destroy" signal here.

	(clutter_actor_init): Sink the initial floating flag if needed.

	(clutter_actor_destroy): Add a function to explicitely destroy
	a ClutterActor.

	(clutter_actor_set_parent), (clutter_actor_get_parent),
	(clutter_actor_unparent): Make set_parent require a valid parent;
	add unparent; check on get_parent; ref_sink the actor when
	setting its parent and unref it when unsetting it.  Probably we'll
	need a function that does reparenting as unparent+set_parent in
	a single shot.

	* clutter/clutter-group.h:
	* clutter/clutter-group.c (clutter_group_dispose),
	(clutter_group_finalize), (clutter_group_add),
	(clutter_group_remove): Make the group destroy its children when
	disposing it; clean up, and use the newly-available
	clutter_actor_unparent().

	* clutter/clutter-stage.h:
	* clutter/clutter-stage.c (clutter_stage_init): ClutterStage is
	a top-level actor; clean up.

	* clutter/clutter-video-texture.h:
	* clutter/clutter-video-texture.c: Clean up.

	* examples/super-oh.c:
	* examples/test.c:
	* examples/video-player.c:
	* examples/test-text.c:
	* examples/video-cube.c: Remove the g_object_unref() call, as the
	ClutterStage object is destroyed on clutter_main_quit().