set title(x)
    {
        if (this._title === x)
            return;
        this._title = x;

        if (x instanceof Node) {
            this.titleElement.removeChildren();
            this.titleElement.appendChild(x);
        } else
          this.titleElement.textContent = x;
    }
@constructor
@param {string|!Element} title
@param {string=} subtitle