forgot to delete this in c8b966b3aa1
turned add_header_vars() into a Header class

Ideally, I'd like to inject those "global" values transparently to the
controller into the view, for example by using a custom twig
environment (meh..) or by injecting them (relatively late, ideally
before rendering something) via some callback setup in
initTwigEnvironment().

Only a few, controller specific things would need to stay there,
like what the currently active navigation item is.