Callback to go to the previous tab. Called by the accel key.

def accel_prev(self, *args):
        """Callback to go to the previous tab. Called by the accel key.
        """
        if self.get_notebook().get_current_page() == 0:
            self.get_notebook().set_current_page(self.get_notebook().get_n_pages() - 1)
        else:
            self.get_notebook().prev_page()
        return True