AcoustID integration

[ianmcorvidae]

I'm not 100% sure what this would entail (probably mostly submission of AcoustIDs, since this isn't a tagger), but as AcoustID is now a fairly core part of the MusicBrainz ecosystem, I think Riker should do whatever's applicable to integrate it.

[kepstin]

If I add support for adding music that's not musicbrainz-tagged to the library, I will definitely include acoustids as a way to look up the music.

Since there's a gstreamer element to do acoustid scanning, it wouldn't be hard to add acoustid submission, I'll look into that.