minor: On teleport, signal a child agent before we send the viewer TeleportFinish in order to avoid a theoretical race condition when teleporting to a neighbour.

If we do this after TeleportFinish, then it's possible for a neighbour destination to request the source to create a child agent whilst its still treated as root.
This closes the original presence which we don't really want to do.
This is probably okay (albeit with warnings on the console) but afaics there's no reason not to move the child agent signal.