Fix MPD handlers
Register new MPD commands and wire handlers across library, playback,
queue and system modules.
Notable behavior changes:
- play accepts an index argument to start playback at a given position
- list artist uses AlbumArtist when requested; added listdate and
listgenre
- queue add starts playback when queue was empty and improves path
handling
- added stop, ping, notcommands, urlhandlers and several queue ops
(moveid,
swap, playlistid, etc.)