Adds support in src/core for tracking override redirect windows.

The metacity core is now directly aware of override redirect windows.
- They get MetaWindows like normal windows, so mutter no longer need to special
  case them. The net wm types applicable to override redirects are also
  recognised.
- The original positioning/stacking semantics of metacity with respect to OR
  windows are maintained, but now it's easier to synchronize the stacking
  between the core and mutter.
- mutter no longer needs to manualy track UnmapNotify, MapNotify,
  ReparentNotify, CreateNotify and DestroyNotify events; instead map, unmap,
  add and remove events can now be consistently delivered via the
  MetaCompositor callbacks.

src/core/stack.h

@@ -49,14 +49,15 @@
  */
 typedef enum
 {
-  META_LAYER_DESKTOP        = 0,
-  META_LAYER_BOTTOM         = 1,
-  META_LAYER_NORMAL         = 2,
-  META_LAYER_TOP            = 4, /* Same as DOCK; see EWMH and bug 330717 */
-  META_LAYER_DOCK           = 4,
-  META_LAYER_FULLSCREEN     = 5,
-  META_LAYER_FOCUSED_WINDOW = 6,
-  META_LAYER_LAST           = 7
+  META_LAYER_DESKTOP	       = 0,
+  META_LAYER_BOTTOM	       = 1,
+  META_LAYER_NORMAL	       = 2,
+  META_LAYER_TOP	       = 4, /* Same as DOCK; see EWMH and bug 330717 */
+  META_LAYER_DOCK	       = 4,
+  META_LAYER_FULLSCREEN	       = 5,
+  META_LAYER_FOCUSED_WINDOW    = 6,
+  META_LAYER_OVERRIDE_REDIRECT = 7,
+  META_LAYER_LAST	       = 8
 } MetaStackLayer;
 
 /**