Don't place Vibrating Square on every special level

Fixes a really dumb oversight that I should have caught in testing.

This is meant to go with commit 2acfdc24b7 (Demaze Gehennom: Remove maze
around Vibrating Square).
Don't put stairs in center of hellfill levels

There might not be much room for it if things like the Wizard's Tower
are taking up space there.
Highlight lit light sources in object listings