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 window blocks the rest of mixxx for me.
Yes, the point is just to avoid that users have extra opened windows.
In this way we force the user to open just one window and also to close it before doing another thing... In other words, much rarely some user will wanna see the cover in a large size, but if they do it, they have to see and close it. Instead of allowing that they have dozens of extra windows which could easily freeze the main thread...