Only force us to stay in hide/replaced if a request happens.

This lets us issue "hide" to a transform by normal means, and override
it later.
Maintain hide_response and replaced_response over all transform renders.

This lets us simply query a transform to determine if it's blocking
hide/replaced, rather than having to update it in an unusual way.
showif: Rewrite showif.

We now simply deliver "appear", "show", and "hide" to the child,
rather than trying to exploit _hide. The advantage of doing it this
way is that we can preserve the hide state going into show, without
having to transfer it between transforms.