webui: reflect enabled state in child widgets of a multivalued widget
    
    Child widgets are disabled when main widget is disabled. Also main
    widget won't show "add" button.
    
    prerequisite for:
    https://fedorahosted.org/freeipa/ticket/4079
    
    Reviewed-By: Adam Misnyovszki <amisnyov@redhat.com>
    Reviewed-By: Petr Viktorin <pviktori@redhat.com>