Add support for reorganizing/retagging tracks etc. in-place

[fwcd]

This could e.g. be useful to eliminate duplicates (merging the best quality version?), merge folders, regroup tracks by a predefined path scheme e.g. ${artist}/${album} etc.

Perhaps we could do this by writing mixync [source] [source], i.e. allowing the source as destination, and perhaps sugaring that to mixync [source] or mixync --inplace [source] (though we'd want to be careful to not perform any destructive operations, so perhaps making a backup would be a good idea?)