constraint: Add ClutterConstraint base class

The Constraint base, abstract class should be used to implement Actor
modifiers that affect the way an actor is sized or positioned inside a
fixed layout manager.

clutter/clutter-types.h

@@ -45,11 +45,12 @@ typedef struct _ClutterStage            ClutterStage;
 typedef struct _ClutterContainer        ClutterContainer; /* dummy */
 typedef struct _ClutterChildMeta        ClutterChildMeta;
 typedef struct _ClutterLayoutMeta       ClutterLayoutMeta;
+typedef struct _ClutterActorMeta        ClutterActorMeta;
 
 typedef struct _ClutterAnimator         ClutterAnimator;
-typedef struct _ClutterAction           ClutterAction;
 
-typedef struct _ClutterActorMeta        ClutterActorMeta;
+typedef struct _ClutterAction           ClutterAction;
+typedef struct _ClutterConstraint       ClutterConstraint;
 
 typedef union _ClutterEvent             ClutterEvent;