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 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
>

This seams to be a uncommon situation for a Album from one Artist.
Normally all of them will get their embedded Album cover at once.
This seams to be most common for samplers or when using single covers.
So You might face the situation that the 6 tracks have a single cover,
because they were released as single. Others have the album cover on
just none like in your case.

conclusion:  as only 6 tracks have cover, he expect to see just them
with a cover