[Do not merge] Also remove embedded art if deleted from the library

[wisp3rwind]

Split out of #17, this PR is relative to #17. This is not really the solution to the problem (see code comment about performance issues), the PR is mostly to track the issue and conserve the code that was written already.

I don't think that this is the way to go because the check for whether artwork needs to be removed currently requires reading metadata from the file in the alternative collection. This is probably (didn't benchmark anything) no faster than simply re-writing all of the tags blindly. So it might be worth considering to simply to tolerate this edge case not being handled (I do not believe that artwork removal is a very common thing to do). One might add a beet alt update --rewrite flag to precisely rewrite tags (including embedded art) for all items in the alt collection.

Another way to handle this might be to use an additional flexattr to track whether the alternative items have embedded art, e.g. item[self.path_key + '.flags'] = 'has_art,'.