Propagate changes in index to the value and label properties

def _propagate_index(self, change):
        "Propagate changes in index to the value and label properties"
        label = self._options_labels[change.new] if change.new is not None else None
        value = self._options_values[change.new] if change.new is not None else None
        if self.label is not label:
            self.label = label
        if self.value is not value:
            self.value = value