write tags with beets instead of exiftool, ffmpeg etc. (in progress!)
The idea is, that we import the music with beets first, but without
moving the files and with a temporary database and default config.
then we can tag everything, and then delete the database again, because
we don't need it anymore and import the files to the real db.

this way we might even solve race conditions, when two imports happen
at the same time (tagging could happen per connected user, disconnected
from the main database, and then the real import would wait in a queue).