Move to MPD

I think it would be best if we replace Mplayer with MPD. One of the advantages is that it features a much better IPC API than the hack we're doing currently with the mplayer subprocess. Additionally the configurability is much better. A challenge will be to handle remote control by 3rd party applications, but I guess we can just ignore that feature and declare it as not supported.

I hope to find time to work on this in February.