XOs (and all icons) in Neighborhood should reposition smoothly

[Eben]

Once we have grouping properly working, and it's in early stages now, we need to make sure that XOs don't jump around the screen seemingly unpredictably. Instead, they should glide across the view into a new position, so that a child observing the mesh sees fluid (or close to it) movement instead of "flashing" XOs. This can happen in surprisingly few frames while still reading clearly.

In the early mockups I used a "naive" ease-out algorithm in the form of:

curr_position += (target_position - curr_position)*k

where k is in range (0, 1), defining how fast it reaches target_position, with 1 meaning immediately and 0 meaning never. It might be better to use a sinusoidal curve instead, so the XO accelerates both into and out of the move, but that might take a few more frames.