Cover art support

- [x] implement database scheme
- [x] search for embedded cover
- [x] search for cover in track directory
- [x] save covers in disk cache
- [x] search for covers in disk cache
- [x] minimal ui (display covers in lower left corner)
- [x] handle existing users
- [x] LRU cache
- [x] unit tests
Hm then the two possible bottlenecks I could think of first is the scaling in wcoverart and sending the pixmap with the signal.

Can you add a ScopedTimer to `void WCoverArt::slotPixmapFound` and report how long this function runs on average.

@cardinot. Can you create a branch where we send a QSharedPointer over the signal instead of the pixmap object. Daniel could then test if this helps the problem

```C++
QSharedPointer<QPixmap> px;
QPixmapCache::find(key