Add %_trackindex% to fix Picard-527 ...

... for e.g. Vinyl whose tracks are numbered A1, A2, ..., B1, B2, ...
Add NEWS.txt
Switch to ~musicbrainz_tracknumber
Merge pull request #251 from Sophist-UK/sophist_add_trackid_variable_Picard-527

Add %_musicbrainz_tracknumber% to fix Picard-527 ...