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 think it is the best to look at the embedded cover art only during the library scan.
We read the metadata there anyway.
Every meta-data read has a potential risk of a segfault, because of corrupt files and bugs in tagreader. 
We have original planed to put all metadata access to a quarantine process. but this project has been sucked.