Cover art support

So this does several things. 

The most important is that I grab an md5 hash from the queued Hash if it contains the trackID. This makes the covers pop up VERY fast. I also update the DB only after 500 tracks are scanned. This make the experience much smoother on my HDD machine.

There is still a freeze at the start that I haven't figured out yet though. Also every 500 tracks mixxx freezes up for a second to save to the database. Still annoying but better then before. Mind that you should update the DB when the coverCache is destroyed, I haven't added that yet.

Oh and the timer now fires every second. We could turn this value up to 10. What you did before was merely delaying the write.

I also turned of the locking. still nicely fast. So I think you can remove that. It would be nice if you can investigate the sqlite update a bit more still. If you can't find a solution we have to update the db in a seperate thread. This would be a hacky solution but make for a nice snappy mixxx until we handle the database stuff in a separate thread.
In the current version this is not needed