Until now the ID3Tag for the Track-Number was read from TPOS.
The Problem is, that TPOS is the part of the set (eg disc number)
and not the track-number. TRCK is the correct tag for the track
number.
Fixes#220
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!