WindowServer: Render the global menubar into a separate WSWindow.

Previously we were rendering the whole menubar on every compose(),
even if nothing changed about it. Now it's in its own window and can
be invalidated and painted separately.

Servers/WindowServer/WSWindow.cpp

@@ -138,6 +138,8 @@ static WSAPI_WindowType to_api(WSWindowType ws_type)
         return WSAPI_WindowType::Taskbar;
     case WSWindowType::Tooltip:
         return WSAPI_WindowType::Tooltip;
+    case WSWindowType::Menubar:
+        return WSAPI_WindowType::Menubar;
     default:
         ASSERT_NOT_REACHED();
     }