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.

Previously, disabling an action would still cause the shortcut sequence defined for that action to be active, which very easily caused clashes and required manual intervention. Now, simply disabling the action will propagate that to the shortcut, resulting in simpler code than the previous state.