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
This starts a search in m_pCoverArtDAO slotCoverArtScan right. 
The control and data flow is hard to follow, can't we call the scan here directly? 
I also think that the location search can be become a own class, It is DAO independent.