Timeline slider should animate incrementally

[avf]

This is something left over from #72, that Justin proposed. I think it's a lot of work so I'm filing a separate issue here:

As you drag the timeline slider, the arrow should go along slightly faster than the actual popup. When you look at the mocks, you can see that, in the 2003 popup, the arrow is completely to the left, and in the 2012 popup, it is completely to the right. It should animate smoothly in between.