CollapsibleChecklists improvements

1. It is always collapsed to start. Consumer should be able to control if a group is expanded or not.
2. ~It uses `<input type="checkbox"`. Implement @paprika/checkbox.~ (this was completed elsewhere)
3. Props are not passed to all descendants of Group... just to direct children (so if you put in any component between Group and Item, checking the Group or the Item will not fire the callback).
