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() and connect() are no light weight calls.
In this case it should be better to use a bool inside updateTrackMetadata() to avoid unnecessary updates.
Are we sure, that we do not ditch valid updates from other threads as well? 
Maybe we need to distinguish the sender before ignore a signal.