[REQ] batch artwork operations

[fishnet37222]

I think it would be really handy for MM to have a feature where you can select multiple albums and apply the same artwork operation to all selected albums. For example, if you have the artwork as external files but want to have it embedded instead. Right now you have to do it manually for each album, which gets very monotonous. I would include operations like extract all artwork to external files, remove all embedded artwork, and embed all external artwork.