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.
showif: Integrate with screen language.

This gives us a showif statement that applies the ShowIf displayable
to its children.
Add updater.UpdateVersion.

This is a function that contacts an update server and determines
if there is an update available. It does this in a background thread,
so as to not make the game unresponsive.
Add the renpy.invoke_in_thread function.
When in self-voicing mode, ignore {w} and {p} text tags.

These (especially the timed variants) tend to disrupt
text-to-speech. It's best to just have them speak in a
single block, whih is closest to the human speech pattern.

Fixes #483.
transform: Apply time adjustments on _hide, as well as render.

Fixes #484.

See: http://lemmasoft.renai.us/forums/viewtopic.php?p=343556#p343556