Allow multilevel dropdown.

Dropdown.php

@@ -84,6 +84,10 @@ class Dropdown extends Widget
 			$linkOptions = ArrayHelper::getValue($item, 'linkOptions', []);
 			$linkOptions['tabindex'] = '-1';
 			$content = Html::a($label, ArrayHelper::getValue($item, 'url', '#'), $linkOptions);
+			if (isset($item['items']) && is_array($item['items'])) {
+				$content .= $this->renderItems($item['items']);
+				Html::addCssClass($options, 'dropdown-submenu');
+			}
 			$lines[] = Html::tag('li', $content, $options);
 		}