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
Ah, it is a direct connection, so I might be wrong. 
I have dived the Qt signal queue some time ago (maybe too long) and what I have leaned is that the signals are stored in a list, able to carry one pointer size data. If we transmit more, it becomes a pointer to a newly allocated buffer. 

You cannot send references by Qt signals. 
http://stackoverflow.com/questions/1935147/argument-type-for-qt-signal-and-slot-does-const-reference-qualifiers-matters

IMHO it is best here to just inform the slot by the track idea and then fetch the pixmap from the cache at the slot.