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
```
3. embedded in file metadata
```

Imagine this case:
- 10 tracks of the same album, stored in the same directory;
- only 6 tracks have a embedded cover;

What would be the user expectancy?
- [ ] as all of them are in the same album, he expect to see all tracks with a cover
- [ ] as only 6 tracks have cover, he expect to see just them with a cover