FormFragment change visibility/hide and remove event listeners should call event.stop() to prevent container fragments from also being hidden

In 5.2.x, FormFragment switched from using direct calls to a "setVisible" function on the fragment to using an event-based approach where the fragment element listens for the Tapestry.CHANGE_VISIBILITY_EVENT or the Tapestry.HIDE_AND_REMOVE_EVENT. The current implementation, however, does not allow for nesting form fragments because the event listeners do not call event.stop(), so the event continues to propagate, so hiding an "inner" form fragment hides the outer form fragment, as well.