Adding a box hit-test

We now have rotating boxes that keep piling up. We need to add a way to remove boxes. The simplest thing would be to add a feature that removes boxes when we touch them, but we are going to make it a little fancier than that: whenever we touch a box, we want it to shrink and disappear before we remove it from the scene. To do this, we are going to use our newly acquired knowledge of components, and then add some code to determine whether we are touching a box.