actor: Add ActorMeta, a base class for actor modifiers

ClutterActorMeta is a base, abstract class that can be used to derive
classes that are attached to a ClutterActor instance in order to modify
the way an actor is painted, sized/positioned or responds to events.

A typed container for ActorMeta instances is also provided to the
sub-classes can be attached to an Actor.

clutter/clutter-types.h

@@ -40,12 +40,16 @@ G_BEGIN_DECLS
 
 /* Forward delarations to avoid header catch 22's */
 typedef struct _ClutterActor            ClutterActor;
+
 typedef struct _ClutterStage            ClutterStage;
 typedef struct _ClutterContainer        ClutterContainer; /* dummy */
 typedef struct _ClutterChildMeta        ClutterChildMeta;
 typedef struct _ClutterLayoutMeta       ClutterLayoutMeta;
+
 typedef struct _ClutterAnimator         ClutterAnimator;
 
+typedef struct _ClutterActorMeta        ClutterActorMeta;
+
 typedef union _ClutterEvent             ClutterEvent;
 
 /**