Register custom 'events' into the DCS Engine Event Handler

[TEMPEST.114]

Rather than a) reinvent the wheel, b) rely on multiple timers and all the possible cross-contamination of those it would be wonderful to have an API into the timer singleton where we can register a custom event and then using the same onEvent handler we have, get notified of when that event fires. 

It would be even more powerful if we could associate some 'logic' into the event we pass in, so that the DCS Engine can evaluate when that's true and fire the event itself.