code: def history_size_changed(self, settings, key, user_data):
          
          lines = settings.get_int(key)
          for i in self.guake.notebook_manager.iter_terminals():
              i.set_scrollback_lines(lines)
docs: If the gconf var history_size be changed, this method will
          be called and will set the scrollback_lines property of all
          terminals open.