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
Please Keep the rules simple: 
Just load "cover.jpg" if you find one, without looking to the size. This seams to be what the user expect. 

folder.jpg is the folder icon in Windows Explorer and not a cover art by definition, this seams to be a good choice before choosing a random image from the folder. 

front.jpg should be preferred over back.jpg.
 
Do not see a reason for album.jpg as plain file name. But we may use a file named %album%.jpg where %album% matches the album tag. This handles the case we have two ore more albums in one folder. 

So finally this priority may work:
```
1. stored path Path in DB  
2. %track-file-base%.jpg in the track directory for %track-file-base%.mp3
3. embedded in file metadata
4. %album%.jpg
5. cover.jpg 
6. front.jpg
7. folder.jpg
8. anything else found in the folder 
```