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/place.c

@@ -394,6 +394,14 @@ rectangle_overlaps_some_window (MetaRectangle *rect,
         case META_WINDOW_DESKTOP:
         case META_WINDOW_DIALOG:
         case META_WINDOW_MODAL_DIALOG:
+	/* override redirect window types: */
+	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:
           break;
 
         case META_WINDOW_NORMAL:
@@ -680,6 +688,14 @@ meta_window_place (MetaWindow        *window,
     case META_WINDOW_TOOLBAR:
     case META_WINDOW_MENU:
     case META_WINDOW_UTILITY:
+    /* override redirect window types: */
+    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:
       goto done_no_constraints;
     }
   
@@ -713,6 +729,14 @@ meta_window_place (MetaWindow        *window,
         case META_WINDOW_TOOLBAR:
         case META_WINDOW_MENU:
         case META_WINDOW_UTILITY:
+	/* override redirect window types: */
+	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:
           if (window->size_hints.flags & PPosition)
             {
               meta_topic (META_DEBUG_PLACEMENT,