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

The tracknumber and the compilation number are now not identical
anymore

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

OggTheora wasn't support up until now. Now the config is there and the
import works

fixes #216