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
coverlocation is the complete path to the cover on disk. We are caching this in the Tableview in `mixxxlibraryfeature.cpp`. to avoid sql queries as much as possible.

This is special because we only need this string to check if we already loaded the cover in the cache and to load it. We could build a special read-only TIO-struct but then this would just be  a reimplementation of TIO which we don't actually need.

I understand why this seems weird to you and I would also like to make this clearer but I don't have any idea how to do this so far.