Updated sidebar to expand to show clicked subtopics, and no sliding.

Here's what I came up with so far, @66eli77, @jtamiace, @rtibbles.
Often one is more appropriate than the other. The main difference I've come across is that custom events are ideal when something repeatable has "happened", whereas listening to model changes is ideal when you want to know when something actually changes (so that repeatedly setting the value on it, if it's the same value, won't trigger the event). Not sure if that makes sense. But yes, using shared model state whenever possible/appropriate seems good.