Option for Soft Limit to apply to children

Problem/Motivation

I noticed on 3504532 that the soft limit feature was intentionally designed to only apply to parent items. This differs from the original facets version of soft limits which applied it to children as well. We are upgrading to facets 3.x and that uses BEF exclusively for widgets now. Our stakeholders are expecting the soft limit to behave the same way as it did before. Right now, our checkbox facets are deeply nested, so limiting the facet to 3 or 5 still shows the entire list of checkboxes. As such, the soft limit is useless to us. We need an option to include children when calculating the soft limit.