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 will do a test on my net-book tonight. 
But for this situation, it is obvious that pointer is much faster. But there might be a lifetime issue. 
You can send "&pixmap" here, because it points to the stack of requestPixmap() which is still valid on a direct connection. But is risky for later changes and a second deep copy is required for saving it. 

So I would feel most comfortable if we go for a QPixmapCache::find(coverLocation, &pixmap) call from the target slot. 
.. Or a own cache implementation using smart pointer guarding the lifetime similar to our track cache.