code: def _new_level(self, depth):
          
          l = self.create_level(depth)
          h = self.create_header(depth)
          self.add_lvl_to_ui(l, h)
          l.new_root.connect(partial(self.set_root, depth+1))
          self._levels.append(l)
docs: Create a new level and header and connect signals
  
          :param depth: the depth level
          :type depth: int
          :returns: None
          :rtype: None
          :raises: None