MenuItem: Add 'parentMenu' property

This is needed since MenuItem is no longer a QQuickItem, and we cannot
access a QObject's parent from QML. Returns 0 for the topmost menu.

Change-Id: I76dd60cb0000066fc1ed675b4767efbcdfc69d0a
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>