require_entity setting: only show items in the hierarchy when they are associated with at least one entity

Title says it all. Merge has requested this as a custom module, but I realized that this is possible to do with the existing HS API, which means that any HS implementation that implements the optional hierarchical_select_entity_count hook can automatically get this feature.

So, instead of writing a custom module, I opted to add a new setting.

Patch is developed against the D6 version, but applied (with offsets) to the D5 version as well, so I rolled a clean patch for D5 as well. Yay for a single codebase!