Append a widget to the list.

        :param widget: Object inheriting :class:`~.widgets.base.BaseElement`

def add_child(self, widget):
        """
        Append a widget to the list.

        :param widget: Object inheriting :class:`~.widgets.base.BaseElement`
        """
        li_itm = _li(id=self.id + str(self._count))
        li_itm.add_child(widget)

        super(List, self).add_child(li_itm)
        self._items.append((widget, li_itm))
        self._count += 1