dirty tracks rarely saved

Currently mixxx relies on a QCache expiration to update the database, but in practice the database is really only updated on exit (when saveDirtyTracks is explicitly called). This means that my crash-helper code doesn't work.

every time a track is played, I toggle the "played" flag. On exit, all those flags are cleared. The idea being that if mixxx crashes, next time it is started the played tracks will already be checked off. Since the database is never updated on my machine except on exit, after a crash I don't have any of the changes that were made to the db.

I'd recommend a 5 minute timer in trackdao that calls saveDirtyTracks, perhaps not making the call if there are analyzer queues or other heavy operations in progress.