more context menu cleanup in video library

As promised I have done some more cleanup in the context menu of the video library. The most important change is commit ce2b63a which moves the "Remove from library" option to the "Manage..." submenu. It is the last cleanup for movies, tvshows, episodes and musicvideos. The other commits add the "Manage..." submenu for movie sets and tags as those also contain multiple items that alter the media item itself.

If requested I can also look into moving the "Choose art" option for tvshow seasons into a "Manage..." submenu but right now it's the only item that would need to be moved there (apart from the duplication of "Mark as watched/unwatched"). I also didn't touch the actor's and artist's "Choose art" option.