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
> can't you just update the picture?
Yes, we could do it...
However, the user could let the new window opened in background and it could be very dangerous...
Some covers can be very heavy and he have to remember that all pixmap will be load in the main thread. So, in this way users could forget to close the window (it's easy to happen, mainly in cases when the user is running mixxx in full screen) and he will never know that the reason why Mixxx  freezing is because he's trying to load all covers unnecessarily in a background window...
Anyway, I don't know if allow it is a good idea... Because most users will never imagine that let extra window opened is a bad idea and it could give them really bad experiences...