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. 
disconnect/connect lock a mutex and add/remove from a list.. if you stand back and squint -- sure it's more expensive than setting a bool. 

Not a big deal for something that is done as a result of the user hitting the "save" or "apply" button on the track edit dialog.. i.e. this is not performance critical code.

Also, if the user already hit apply, they are expecting the data in the dialog at that moment to become the new track data. Updates from other threads should be overruled in this case since the user just explicitly approved what was in the dialog.