HtmlHelper to generate a top-level menu for areas

Last week, we had to set up a new ASP.NET MVC web application, using a somewhat customized Twitter Bootstrap build. Because the application has multiple functional contexts, we divided it in multiple parts using areas. Since these areas were a one-to-one mapping with the top-level menu items, we tried abstracting the creation of the menu items, ánd the management of setting the active item, into an HtmlHelper.