much faster bulk-select on filtering multiselect

@camd r?

I'll explain inline. 
The way this works is that an event is hanging on every checkbox and when it changes it determines if there's any checkbox within the container that is not checked. If there is at least one that isn't checked, it unchecks the check-all one (aka. the bulk select) so that you can click it again to get all checkboxes checked. 
The way it does this is very very expensive. It only takes 2-3 milliseconds but when it's done thousands of times it matters greatly.