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
It means, something like:
``` c++
CoverArtDAO::coverArtInfo coverInfo;
if (!QFile::exists(coverLocation)) {
     CoverArtDAO::coverArtInfo coverInfo = m_pCoverArtDAO->getCoverArtInfo(trackId);
}
```
As all covers that were loaded correctly once would be stored in our disk-cache, it would return true most of times and we would be doing the db query just when it would be useful for loadImage()...