MetaStream: big clean-up, implement play statistics methods

* remove many unused and useless methods such as setTitle() and
friends
* additionally take genre, comment, track number from Engine
Controller's signal
* implement rating, score, first & last play and play count. These are
rather debugging tool for EngineController, but they also give you
nice info on how many songs have played in the stream
* no need to reimplement observer pattern, use MetaBase implementation
* implement length, EngineController will emit length info soon