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
> Hey, just keep in mint that there is a pending pull request from last year, that moves all db quires into a dedicated thread.

But regardless of that the code will still be easier to understand if we don't do everything in 'loadImage' and we would benefit from it either way.