force lines to be black

more consequences of the 086b2fe221a2aca5a5a47411815d81704fdc12fa bug.
slow down player further

I was surprised that it was still considered too fast. That's what user
testing is for!
giving credit where credit is due.
inter-level hints
the door is no longer part of a wall.

user testing indicated that the old way was weird.
I still think that if walls can appear out of nowhere, walls containing
doors are just as likely...
prevent key duplication (except as designed)
highlight stage changes by blinking them