NScumm.Audio.Players should support stopping audio

[BenMcLean]

The way the NScumm.Audio.Players API currently works is that songs are assumed to be playing all the time. You can call Load again to change to another song but there's no option to stop playback. I recommend adding a Stop method to the API.

[BenMcLean]

I suppose this isn't strictly necessary since outside code with access to the IOpl instance could achieve the same effect by manually sending a "mute" signal and refrain from calling Update() when music is supposed to be stopped, but it seems like implementing a stop function would be cleaner.