Nestable checkbox/radio widget
    
    New component: option_widget_base. It's not a regular widget but it share some of its characteristics. It should extend regular widget or it can be nested in itself alone.
    
    checkbox_widget, checkboxes_widget, radio_widget were modified to use it.
    
    Built as a prerequisite for:
    https://fedorahosted.org/freeipa/ticket/3404