Move getProperties and get/setUserData up to MenuItem

Anything that extends from Node inherits a properties map, as well as a userData property for storing and later retrieving useful properties. Because RadioMenuItem is a Toggle, it requires this functionality (as Toggle defines these methods in its interface). However, because MenuItem is not a Node or Control, it does not have this API. Currently, the API is only implemented in classes that extend from Toggle, but for the sake of consistency, it should be moved up to MenuItem.