Lazily evaluated neighborhoods for node triggers

[clayote]

Currently, node triggers are evaluated for every node in their category, which may take a while.

If triggers were only reevaluated when their node changed somehow, they would be faster. But a lot of triggers consider nodes beyond the one they are specifically attached to.

Neighborhoods on triggers would let the user declare that the trigger is interested in nodes that are close to the one they are attached to, so that the trigger is only evaluated when the node in question, or ones connected to it at so-many removes, are changed.