Implement \hidden/\hide as a shorthands for \tweak/override #'\stencil = ##f (issue 6443087)

[graham]

I'm really not happy with \hide vs. \hidden depending on whether it's
override or tweak.  How do we expect novice users (or moderate users) to
know which they need to use?

http://codereview.appspot.com/6443087/