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
Ah, thank you for explanations. OK, bad idea, using the TIO. 

What is the content of coverLocation, is it a path?
Today you send the "coverlocation" in future  you may need other track properties as well. What makes  "coverlocation" 
special?  

It works and it seems to be performant so it is fine. 
But I am still struggling with this, because we are here in the view class of the MVC-pattern. 
Is it possible to provide a way to fetch properties from the focused track from the model? 

If you think this is not worth the work, it is OK. But once we need other track properties for things like text overlays we may re-think this. Should we place a comment here for this case?