Tweak animation for broadcast messages#448

Currently when two broadcasts appear in quick succession, the newer one is shown higher up, above the older one, which is slid down. The problem is that not only is this the reverse of how they show up in the chat, the animation for them appearing shows them flying in from the bottom, so it breaks not only prior knowledge, but momentary intuition (not to mention making things hard to read as messages have to slide through eachother)

the fix? just have messages slide up as more are added instead of down, which fixes all the above problems.