actor: Make Actor instantiatable

ClutterActor now has all the API and capabilities for being a concrete
class:

  - layout management, through delegation
  - container implementation and API
  - background color

This means that a simple scene can be built straight out of actors
without using subclasses except for the Stage.

This is the first step towards the deprecation of most of the Actor
subclasses provided by Clutter.

clutter/clutter-actor.h

@@ -261,7 +261,9 @@ struct _ClutterActorClass
   gpointer _padding_dummy[28];
 };
 
-GType                 clutter_actor_get_type                  (void) G_GNUC_CONST;
+GType clutter_actor_get_type (void) G_GNUC_CONST;
+
+ClutterActor *        clutter_actor_new                       (void);
 
 void                  clutter_actor_set_flags                 (ClutterActor          *self,
                                                                ClutterActorFlags      flags);