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 even worse. You are writing here. This is definitely blocking the database because it needs to do a real write to disk and some consistency checks. This is an open invitation to weird race-condition bugs