Make WheelPane handle varying node sizes.

* First, set the radius to be large enough to fit all nodes
  along the circumference (as measured by the bounding box diagonals),
  and to fit the largest two nodes in a line with the center (so that they
  can be opposite each other if needed).
* Next, give each node a proportion of the circle that matches their diagonal
  in proportion to the sum of all diagonals.

The WheelPaneExample now uses circles of different sizes, showing that the space
between nodes is roughly maintained even as the nodes grow larger.