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
>I think it is the best to look at the embedded cover art only during the library scan.
>We read the metadata there anyway.

We did it in first implementations... (before this commit f8cb2843af83861dfef96a3f4825330d7d735253)
The problem of doing the cover extraction during the library scan is that it makes the scan much slower and it could bring very bad experiences mainly for a user with a very big library.
Also, getting covers just during the scan, means that the existing users would be forced to rescan their library to start to see covers...

The current concept is to load covers in a **on-demand** way. It means that we will load just the covers that the user wanna see. In this way, covers will start to appear automatically (for new or old users), without having to use/click in any new option/button...