Implement filter "Don't show items that won't narrow down the results"

This request is a duplicate of #1234894: Implement filter "Don't show items that won't narrow down the results", because it has become clear this is the dominant 'extra features for Facet API' module.

Basically all I want is to be able to filter out any facets that won't narrow down the results - that is, they exist on all entries in the current result set.
Obviously it shouldn't hide facets that are already selected.