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
Imagine a trackDirectory with the following files:
```
1 - album.jpg (300 kb)
2 - front.jpg (100 kb)
3 - cover.jpg (900 kb)
4 - folder.jpg (200 kb)
```
Which of them would you prefer to get?