Due to evaluation order, menus can be appended to the default property
before the native property is set => sync the items as appropriate.
Technically it's now even possible to toggle between a native and
non-native menubar even though it doesn't sound like a sane use-case.

Change-Id: I513f133a085ffd980a4000afe2a992cb47b7448e
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>