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
coverLocation seams to be optional. Swap the parameter and make coverLocation default to an empty string. 
By the way, it is a good practice to pass string by constant references:
const QString& coverLocation;