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
I would prefer sending a pointer to the pixmap then. It is easier in the classes that are listening to this signal when it also delivers the image right away.

I'm not sure if this is a performance bottleneck because I haven't notices anything on my machines. Can you try it on your netbook and report if you are seeing performance issues.