New phrases for intra- and inter-world relations

This patch adds the following phrases:
1. A "denizen" is someone who lives in the main dungeon.
2. "A and B share a world" if they are in the same dungeon (persons also
need to be alive).
3. Something is a "worldsharer" if it shares a world with "world test
subject". Set the value of "world test subject" before using this
phrase.

All extensions have been searched for occurrences of the word
"off-stage," and may changes using the new phrases have been made.

Closes #119.