bug #1349017 - DlgTrackinfo and DlgTagFetcher should be updated together after clicking on PREV or NEXT buttons

Test case:
- open dlgtrackinfo (properties)
- open dlgtagfetcher ("get metadata from MusicBrainz")
- hit PREV or NEXT in the dlgtrackinfo
   -> expected: dlgtagfetcher load the same track
   -> actual: it keeps frozen in the initial track

https://bugs.launchpad.net/mixxx/+bug/1349017