Create a new level and header and connect signals

        :param depth: the depth level
        :type depth: int
        :returns: None
        :rtype: None
        :raises: None

def _new_level(self, depth):
        """Create a new level and header and connect signals

        :param depth: the depth level
        :type depth: int
        :returns: None
        :rtype: None
        :raises: None
        """
        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)