Fix chat fade bug.

Here's the rub: we were doing entirely too much manual chugging and
plugging on the chat fade status, so some corner-cases slipped through. In this case, we absolutely DO NOT WANT to start the fade timer if the user has entered any chat (self.chatMgr.isChatting). We were doing just that any time the chat
history display was modified (oops!).

See also:
- http://forum.guildofwriters.org/viewtopic.php?f=117&t=6076
Smoother chat fade...

... The old one was rough like sandpaper and looked like crap. This is
much nicer.