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/keybindings.c

@@ -862,7 +862,8 @@ meta_window_grab_keys (MetaWindow  *window)
   if (window->all_keys_grabbed)
     return;
 
-  if (window->type == META_WINDOW_DOCK)
+  if (window->type == META_WINDOW_DOCK
+      || window->override_redirect)
     {
       if (window->keys_grabbed)
         ungrab_all_keys (window->display, window->xwindow);