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 do not like the idea of saving covers in a mixxx private disk-cache. This will just contain duplicate copies and will pollute the HD. Is there significant speed improvement if we fetch a cover from a dedicated file compared to a meta data read?  
Even for downloaded covers, I personally would prefer to store them along with the tracks. This way they are available for other apps as well. But this should be a preference option. .. and we must handle the case that the disk is full.