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.