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/core.c

@@ -145,6 +145,13 @@ meta_core_get (Display *xdisplay,
             case META_WINDOW_DOCK:
             case META_WINDOW_TOOLBAR:
             case META_WINDOW_SPLASHSCREEN:
+	    case META_WINDOW_DROPDOWN_MENU:
+	    case META_WINDOW_POPUP_MENU:
+	    case META_WINDOW_TOOLTIP:
+	    case META_WINDOW_NOTIFICATION:
+	    case META_WINDOW_COMBO:
+	    case META_WINDOW_DND:
+	    case META_WINDOW_OVERRIDE_OTHER:
               /* No frame */
               base_type = META_FRAME_TYPE_LAST;
               break;