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
>>Do you think that we should reorder it?

>yes always prefer metadata. Lets just make this a design rule. This is also the intended way to save a cover per track in every other application that I know. Plus id3v2 was made exactly for that purpose.

@daschuer, if it's also fine for you... c7d07c98dacab15380dfa58ec1e71da2db520416