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! Especially with the highly parallel asynchronous Qt:Concurrent system. It is too easy for this to introduce race-condition bugs. I'd rather have that we avoid them when we can.