Get the number of displays and populate this drop-down box
        with them all. Prepend the "always on primary" option.

def populate_display_n(self):
        """Get the number of displays and populate this drop-down box
        with them all. Prepend the "always on primary" option.
        """
        cb = self.get_widget('display_n')
        screen = self.get_widget('config-window').get_screen()

        cb.append_text("always on primary")

        for m in range(0, int(screen.get_n_monitors())):
            if m == int(screen.get_primary_monitor()):
                # TODO l10n
                cb.append_text(str(m) + ' ' + '(primary)')
            else:
                cb.append_text(str(m))