Use the keyboard to control selection of the buttons.

def __update_keyboard(self, milliseconds):
        """
        Use the keyboard to control selection of the buttons.
        """
        if Ragnarok.get_world().Keyboard.is_clicked(self.move_up_button):
            self.move_up()
        elif Ragnarok.get_world().Keyboard.is_clicked(self.move_down_button):
            self.move_down()
        elif Ragnarok.get_world().Keyboard.is_clicked(self.select_button):
            self.gui_buttons[self.current_index].clicked_action()

        for button in self.gui_buttons:
            button.update(milliseconds)