bugfix #58: Check custom action usage inside other custom actions by using a queue

[NoelTautges]

This PR fixes bug #58 by using a queue to check custom action usage.

Beginning with the main body of the Shortcut, custom action uses are detected and added to the queue. The function continuously cycles through the queue, adding newly used custom actions, until all possible matches are exhausted.

Notes:

• It makes sure the action isn't already marked as used because otherwise recursive functions go haywire.
• It adds a dummy custom action to make the loop that goes through the queue more clean. This is... questionable, but it does work.
• Ignore the PR behind the curtain (Check custom action usage inside other custom actions by using a queue #59), I accidentally closed it after syncing my fork with the upstream repository.

[electrikmilk]

Thanks so much!