Add option to disable sub-items if parent is checked

I've updated the patch to also show the child elements as checked and also replaced the control_id to use the attribute "data-drupal-selector" instead of "id". This way the code also works if the widget is reloaded with ajax.

I've updated the patch to prevent triggering change on child elements when "parents only" option is enabled otherwise it renders the widget almost unusable when ajax is enabled and the item has many children.