videolibrary: add setting to not group movies into a set if it contains a single item

This functionality has been requested ever since movie set support has been added to TheMovieDb and the scrapers which results in a lot of movies being part of a set. The problem is that a lot of sets only contain a single movie either because the other movies belonging to that set are not in the library or because there are no other movies belonging to that set yet (but are already anticipated).

These sets are hidden in the movie list (i.e. we show the movie and not the set) but that logic doesn't exist in the Movie Sets view. There, depending on the use case, it may make sense that we should only show sets that contain more than one movie or to show all sets to be able to manage them by hand.

Ideally that toggle would only be available in the Movie Sets view but IMO that's too much specific work for this use case so I added a global setting which will affect both sets in movie listings and sets in the Movie Sets view.