Cover art support

- [x] implement database scheme
- [x] search for embedded cover
- [x] search for cover in track directory
- [x] save covers in disk cache
- [x] search for covers in disk cache
- [x] minimal ui (display covers in lower left corner)
- [x] handle existing users
- [x] LRU cache
- [ ] unit tests
yeah `trackChanged` doesn't seem to do much for what we want. I've never checked the code much for this and I'm a bit surprised that it works with TIOs. But there still is a function in the BTC that you can use `updateTrackInIndex`, this is private so you have to call it with a signal. TrackDAO so far uses the `tracksRemoved` and `tracksAdded` signals to do that.  Write you own signal `updateTrackInBTC` and connect it to a new slot `slotUpdateTrack` which then calls `updateTrackInIndex`. This should work without TIO's