This equalises the API with QQC2's actions (which also disable shortcuts when the action itself is disabled)
nb: This pushes the task of disabling actions on sub-pages and anywhere else not currently supposed to be active onto the app developer, but it reduces complexity a very great deal, and also means when it comes time to switch to QQC2.Action. It is less magic, but this is not necessarily a bad thing. It would still be nice to have everything on non-current pages entirely disabled when the page is not current (or indeed visible), but this works.