MenuButton, SplitMenuButton, ChoiceBox needs onShow* and onHid* events

MenuButton, SplitMenuButton, ChoiceBox needs onShow* and onHid* events
Justification: Any control that displays a context menu should consistently provide the events pre- and -post- showing and hiding the menu. This is a common request from developers, and whilst this API exists in some controls, it is not in all of them, resulting in an inconsistent API.