Allow composer to disable standard application keyboard shortcuts

If a composer uses a Receive Keyboard Buttons node configured to fire events when a standard keyboard shortcut (such as cmd+Q) is used, the standard behaviour should be disabled.

Alternatively, if the above is not implemented, the composer should receive a warning of some flavour if he or she attempts to use an existing keyboard shortcut in a Vuo composition.

The first option, disabling the standard shortcuts when they’re overridden, sounds good. Opened for voting.