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
This is not good. You are calling a functions across threads and pass data between them. This could end that you potentially do really a LOT of parallel queries. I don't know how sqlite deals with that