Allows extensions with custom nav-item to mark "Extensions" active

Extensions can add a navigation-item by calling ```$this->addMenuOption``` and display their extensions' views under a specified route.
Not sure if this is the most elegant way to proceed, but this patch enables extension-authors to to mark "Extensions" as active in the backend navigation

```
{% extends "_base/_page-nav.twig" %}
{% block page_nav 'Settings/Extensions' %}
```