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/window-private.h

@@ -110,6 +110,9 @@ struct _MetaWindow
   /* Initial timestamp property */
   guint32 initial_timestamp;  
   
+  /* Whether this is an override redirect window or not */
+  guint override_redirect : 1;
+
   /* Whether we're maximized */
   guint maximized_horizontally : 1;
   guint maximized_vertically : 1;