Revise handling of album art

[edmw]

Currently album art is cached by cover image url. If the album art url is not provided by the Volumio player this results in a request to last.fm api first. In my environment this has two effects: first, there are a lot of requests to last.fm api (which are hopefully cached by URLCache). Second, there is always a small flicker of the album art images in the table views (queue and browser).

I did some research and think we can resolve this if we cache the album art by album name and artist name (which are the two parameters which will be used to make a request to last.fm api).