transcode: Improve copying tags

This cleans up and improves the copying of tags. All tags are first
converted to ID3 for further processing (like loading folder.jpg or
applying hacks).
hack: Add disc number hack

Some media players don't properly support disc number tags with tracks
numbered starting with 1 for every disc. The user typically wants to group them
by disc and not by track position. This can be solved by creating a different
album for each disc. With the `--discnumber-hack` option, the disc number is
appended to the album field.
hack: Add track number hack

Some media players don't properly support track number tags containing
the total number of tracks on the disk. With the `--tracknumber-hack` option,
the track total is removed from the track number field.