Return newly added text box shape appended to this shape tree.

        The text box is of the specified size, located at the specified
        position on the slide.

def add_textbox(self, left, top, width, height):
        """Return newly added text box shape appended to this shape tree.

        The text box is of the specified size, located at the specified
        position on the slide.
        """
        sp = self._add_textbox_sp(left, top, width, height)
        self._recalculate_extents()
        return self._shape_factory(sp)