Refactor Meta::Observer so that we automatically unsubscribe all Meta subscriptions in the dtor.

This fixes many issues, e.g. you are now actually able to listen to more than 5 tracks in a row without crashes ;)

Also, it makes the code simpler, since we don't have to do manual bookkeeping.

The important API change is that you now subscribe/unsubscribe to Meta objects with subscribeTo(MetaPtr) / unsubscribeTo(MetaPtr); these two methods are built into the Observer base.

This was quite a bit of work, but I think it was worth it :)