Refactored the integer id->ClutterActor mapping code to be a self

contained data structure.
* clutter/clutter-id-pool.[ch]: new files.
* clutter/Makefile.am: added clutter-id-pool.[ch]
* clutter/clutter-private.h: use a ClutterIDPool instead of GArray and
GSList to keep track of the reusable ids.
* clutter/clutter-actor.c: moved id pool logic away.
* clutter/clutter-main.c: simplified id pool creation/finalization.

clutter/clutter-private.h

@@ -44,6 +44,7 @@
 #include "clutter-backend.h"
 #include "clutter-stage.h"
 #include "clutter-feature.h"
+#include "clutter-id-pool.h"
 
 G_BEGIN_DECLS
 
@@ -83,8 +84,7 @@ struct _ClutterMainContext
   guint            motion_frequency;   /* Motion events per second */
   gint             num_reactives;      /* Num of reactive actors */
 
-  GArray          *actor_array;        /* Array of ClutterActors        */
-  GSList          *free_actor_ids;     /* A stack of released actor ids */
+  ClutterIDPool   *id_pool;            /* mapping between reused integer ids and actors */
 
   guint            frame_rate;         /* Default FPS */