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
Performance reasons. Creating a TIO is actually a lot of heavy work, we actually did it with a TIO in the beginning checkout c1a2d58. Even if you have all covers cached in memory already loading a lot of covers quickly will stall the UI-thread. It is a workaround for the fact that we do our db queries in the UI-thread. 

Covercache also handles everything if the `coverlocation` is invalid and not set yet.