layout-manager: Add a private struct to hold future properties

In order to allow the abstract ClutterLayoutManager class to have own
GObject properties we'll need a private structure.

https://bugzilla.gnome.org/show_bug.cgi?id=676827

clutter/clutter-layout-manager.c

@@ -345,6 +345,11 @@
                    G_OBJECT_TYPE_NAME (_obj),                           \
                    (method));                           } G_STMT_END
 
+struct _ClutterLayoutManagerPrivate
+{
+  gpointer dummy;
+};
+
 enum
 {
   LAYOUT_CHANGED,
@@ -585,6 +590,8 @@ clutter_layout_manager_class_init (ClutterLayoutManagerClass *klass)
   quark_layout_alpha =
     g_quark_from_static_string ("clutter-layout-manager-alpha");
 
+  g_type_class_add_private (klass, sizeof (ClutterLayoutManagerPrivate));
+
   klass->get_preferred_width = layout_manager_real_get_preferred_width;
   klass->get_preferred_height = layout_manager_real_get_preferred_height;
   klass->allocate = layout_manager_real_allocate;
@@ -638,6 +645,9 @@ clutter_layout_manager_class_init (ClutterLayoutManagerClass *klass)
 static void
 clutter_layout_manager_init (ClutterLayoutManager *manager)
 {
+  manager->priv =
+    G_TYPE_INSTANCE_GET_PRIVATE (manager, CLUTTER_TYPE_LAYOUT_MANAGER,
+                                 ClutterLayoutManagerPrivate);
 }
 
 /**