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

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?)