Some cover art changes.

Hey @cardinot, @kain88-de,

Great work with this feature! It's super fun to use. 

I made some changes -- I realized the set of things I was asking you to do were basically unreasonable to come to you this late in the game and ask for so I went ahead and did them myself this weekend. 

I know I made some significant changes to the design but I think they were necessary. I'm happy to discuss any of them. 

I'm really excited to have this in Mixxx 1.12.0. I think the branch is merge-able with these changes. There are some improvements I want to do (I left myself TODOs) but I think they're fine to do post-merge. 
*really nitpicking here*

Can you change the comment so that it is clear the file exists to test the we don't choose random covers if several image files exists and none of them fits our algorithm. Thanks