Extendable Headlined View Model

ExtendableHeadlinedViewModel<TSelfInterface> is an abstract view model that derives from ViewModel<TExtensionIfc> — passing TSelfInterface as the extension interface — and implements IHeadlinedViewModel, adding a header, icon, description, visibility and order.

It is a sibling of HeadlinedViewModel rather than a subclass: both implement IHeadlinedViewModel, but HeadlinedViewModel builds on the plain ViewModel base, while this class builds on the extensible one. The type parameter is constrained to where TSelfInterface : class, ISupportId<NavId>.