Update Tagfetcher about changes in TrackInfo and vice versa

https://bugs.launchpad.net/mixxx/+bug/1349173

This also updates the API of dlgtagfetcher to be similar to the dlgtrackinfo API
While this seams to work, I wander if it is better to pipe this "Updated" signal somehow though the track itself.  There might be other places in Mixxx which may change the track meta data or may interested in the update signal.

It would also be nice, to make it independent from the a single wtracktableview.cpp. We may have two of them in future.