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. 
Yea, QRegExp is not thread safe which is why I didn't make a static one. The places where there is a repeatedly used QRegExp all pre-allocate it. guessCoverArt is the only one that allocates one on the fly and that is currently only used for one-off reloading of the cover. That should eventually move out of hte main thread and into a background worker or something so at that point I would get rid of it.