Selects an item by its key.

        Args:
            key (str): The unique string identifier of the item that have to be selected.

def select_by_key(self, key):
        """Selects an item by its key.

        Args:
            key (str): The unique string identifier of the item that have to be selected.
        """
        self._selected_key = None
        self._selected_item = None
        for item in self.children.values():
            item.attributes['selected'] = False

        if key in self.children:
            self.children[key].attributes['selected'] = True
            self._selected_key = key
            self._selected_item = self.children[key]