Extendable Tree Subpage

ExtendableTreeSubpage<TContext, TSubContext> is the extensible counterpart of TreeSubpage: both implement ITreeSubpage and expose the same Menu/MenuView surface, but this one derives from ViewModel<TExtensionIfc> instead of the plain ViewModel base, so extensions can be applied to it.

Like TreeSubpage<TContext>, it receives its ITreeSubPageContext<TContext> through the constructor, which forwards the navigation arguments to the base view model.