playbackStateChangedListener/more event emission

[gossi]

At the moment it is not very easily possible to get notified when playback has been started or paused. There is the chance to registerCallback but that would notify on each update. Would be nice to have more event emitted, to properly build stuff around, .e.g. to register a callback whenever state changes.

[PTaylour]

Sounds good.

We're about to merge some better regression testing. Adding more events could be a good change to get in after that