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
Hey, just keep in mint that there is a pending pull request from last year, that moves all db quires into a dedicated thread. 
I hope we can merge it in the 1.13 release, so there is no need to worry that much here to use the DB from the GUI thread.

And yes, db queries can involve HD access. I just wander if it is required to cache all our date in ram, rather than using sqlite cache facilities, but that is an other topic. (I am no sqlit expert)