Can now import covers from track metadata and track directory as well

The tracknumber and the compilation number are now not identical
anymore

If you ever need an empty flac file with metadata again:

1. Get a flac file (like https://archive.org/download/NineInchNailsTheSlip24bit96khz/01999999.flac)
2. Tag it with Musicbrainz Picard
3. Truncate it, keeping only tags with `ffmpeg -i in.flac -ss 0 -to 0.001 out.flac`

Thanks @HgO for the trick!