Crossfade songs

I want to have an option to cross-fade between songs. The current implementation pauses the current playing track and starts the new song. The new implementation should wait until the next song has enough data to start playing (check the buffer of the player) and slightly taking down the volume of the currently playing song while start playing the new song by slightly increasing the volume.

It should be possible to:
- Start buffering the song way before the cross-fading starts of (because it may needs some time to load)
- Set the seconds for when the cross-fading should start

Please include this in a test and decide what to do:
- What to do if a track is shorter than 2x the time of cross-fading (fading in and fading out)

Maybe it's worth thinking of opening up for a queue, that should just be an instance implementing `IEnumerable`. The programmer can decide if he wants to have a simple list like `new List()` or a more advanced implementation, including shuffling, repeating, etc - but that's up to the integration then.