TODO for the next engine framework:

 * emit stateChanged() with no parameter and force controller to read the state from
   EngineBase::state() instead, as this makes things more likely to be consistent
 * combine load() and play() as it just complicates things for engine development (eg
   crossfade is a bitch for me due to the separation
 * setVolumeSW( double ), to give more resolution for setting volume
 * Make EngineObserver system use engineStateChanged( oldState, newState )
 * Don't do above function for Play on every track? Is this ever useful?
 * Would be useful if engineNewMetaData and stateChanged for Play weren't separate?
 * setVolumeSW uses a log function, volume() doesn't so you get different values back to
   what you expect. Really we should provide a volumeToLog( int ) function for engines to
   use internally as some engines may already apply a log function anyway.