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
> Did you check BaseTrackCache, you can tell it that some tracks changes and reload the information for these tracks into the tablemodels.

I tried emitting **trackChanged(trackId)** in **TrackDAO::updateCoverArt** but it was not enough. It seems that we are not including cover_art in this track update...
Maybe I'm wrong but seems that we will update the tablemodel from the TrackPointers, isn't it?