An implied goodbye topic. This handles ONLY automatic (implied) conversation endings, which happen when we walk away from an actor we're talking to, or the other actor ends the conversation after being ignored for too long, or the other actor ends the conversation of its own volition via npc.endConversation().

We use a higher-than-default matchScore so that any time we have both a ByeTopic and an ImpByeTopic that are both active, we'll choose the more specific ImpByeTopic.

class ImpByeTopic :   MiscTopic