Respect 'Limit to one active item'

I'm currently using this widget as it appears to be the only one capable of displaying a taxonomy hierarchy correctly (all the others ignore child terms unless the list is flattened, at which point there's no visual distinction between parents and children).

I'm fully aware that the module is explicitly named "Multiselect", but I think it would make sense to actually make that part optional, given there's already an option to limit the facet so only one term at a time is used.