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-constraint.c

@@ -0,0 +1,21 @@
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include "clutter-constraint.h"
+
+#include "clutter-actor-meta-private.h"
+
+G_DEFINE_ABSTRACT_TYPE (ClutterConstraint,
+                        clutter_constraint,
+                        CLUTTER_TYPE_ACTOR_META);
+
+static void
+clutter_constraint_class_init (ClutterConstraintClass *klass)
+{
+}
+
+static void
+clutter_constraint_init (ClutterConstraint *self)
+{
+}