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
> no Db queries outside of the UI-Thread!

But my suggestion is move it to requestPixmap() to run in the main thread... so, the code that I suggested was to be placed to run in the main thread...