code: def populate_display_n(self):
          
          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))
docs: Get the number of displays and populate this drop-down box
          with them all. Prepend the "always on primary" option.