Only render icons of root navigation items

The upcoming navigation dashboard creates its links from the menu items in configuration.php.
Now, we encourage users to specify icons for second-level menu items but do not want render them in our menu.

library/Icinga/Web/Navigation/Renderer/RecursiveNavigationRenderer.php

@@ -162,6 +162,9 @@ class RecursiveNavigationRenderer extends RecursiveIteratorIterator implements N
         foreach ($this as $item) {
             /** @var NavigationItem $item */
             if ($item->shouldRender()) {
+                if ($this->getDepth() > 0) {
+                    $item->setIcon(null);
+                }
                 if ($this->getUseStandardItemRenderer()) {
                     $renderer = new NavigationItemRenderer();
                     $content = $renderer->render($item);