new styles are set.
Tying transitions to the DOM node avoids quadratic complexity when
updating them.
Finishing transitions instantly when styles are updated makes our
behavior more correct.
Transition events are not yet supported, and the only animatable
properties are `top`, `right`, `bottom`, and `left`. However, all other
features of transitions are supported. There are no automated tests at
present because I'm not sure how best to test it, but three manual tests
are included.
