Add sanity checks for controller assignments

[pahund]

In the track definition JSON file, a track def author may mistakenly assign the same controller for the same MIDI bus and channel to multiple channel elements or to playing.

In this case, multiple MIDI controller commands are sent to the same channel when the scene changes, causing unpredictable results.

Add a validation step that checks for mistakes of this kind and throws an error.

Another possible solution for this may be that in some cases, this is allowed, see issue #59.