webui: fix regression: enabled gid field on group add
    
    GID field should be enabled by default since the default group is posix.
    
    Was caused by option_widget_base not properly reporting value change while
    selecting the default value. It has to be notified with delay otherwise the
    event is consumed by FieldBinder.
    
    https://fedorahosted.org/freeipa/ticket/4325
    
    Reviewed-By: Endi Sukma Dewata <edewata@redhat.com>