Cover art support

- implement database scheme
- search for embedded cover
- search for cover in track directory
- save covers in disk cache
- search for covers in disk cache
- minimal ui (display covers in lower left corner)
- handle existing users
- LRU cache
```
2. %track-file-base%.jpg in the track directory for %track-file-base%.mp3
```
Do you mean **track_location.filename**?