Fix animations when switching between chat tabs

[mixxorz]

Currently, when the tab switches and the other chat frame appears, messages appear as if they've only just arrived (they slide up). Instead, they should just fade in. Moreover, the current chat frame should fade out while the other fades in.

[mixxorz]

A possible way to address this would be to replace the OnUpdate handler on SlidingMessageFrame to use NewTicker instead. This way, updates to the frame are handled even if the frame is hidden.

While doing this ticket, we should also make it so that when a chat tab is clicked, its hidden messages become visible.