Bug 1092327 - Add a press state for the collapse toggle on groups. r=kgrandon

The toggle doesn't transition when you add the toggling class, only when you remove it (so that the response is immediate - there's still a delay and I found it was really the only acceptable and natural-feeling way to go about it) - I guess we could add a very short transition for the toggle, but I think this ends up being less work and so has less chance to introduce jank.

We could probably replace the setTimeout with a twice-nested requestAnimationFrame, but there's a chance in that situation that it draws one more frame than is necessary (which isn't really a big deal I suppose, but I think nesting two requestAnimationFrame's may look even uglier than the setTimeout and may end up being more computationally expensive).

The short of it is though that we want this inner block to execute as soon as possible, but *after* the toggle has responded to the user. I thought that rAf would do it, but the comment details what happens in that situation :(