ChatView now asks the scene if scrolling is ok on message appending

ChatView now asks the scene if scrolling is ok on message appending

That way, the scene can inhibit autoscroll for example while selecting, or during
a click sequence (nothing is more annoying than a link that scrolls away while you have
the button still pressed...). The former is already implemented, the latter follows next
as soon as I get a decent click handling going.