public static function add($title = null, $url = ['action' => 'add'], $icon = 'plus', array $options = [])
    {
        $toolbar = Toolbar::getInstance(self::$_toolbar);
        $options += [
            'icon'   => $icon,
            'button' => 'green lighten-2'
        ];

        $toolbar->appendButton('Core.link', $title, $url, $options);
    }