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
maybe rename `currentCoverLocation` to `dbCoverLocation` or similar, to make it clear that this string contains the location that is saved in the database and not constructed from the artistsname/album. I just thought you had a bug because I forgot it and had to check `m_pCoverArtDAO->getCoverArtInfo(trackId)` again.