Bugfix in WheelPane: Remove old connections.

The wheelpane didn't clear its old springs, which
would cause severe animation lag after many layout passes,
and also made the simulation behave incorrectly.
Randomize behavior of point-compressed springs.

In practice, nodes are often initially placed on the same
point, making their spring connections inactive.

Instead, if two endpoints exactly coincide, the spring will now behave as
if they were infinitesimally displaced in a random direction, and will not
get "stuck".
WheelPaneExample dynamically adds/removes circles.

Just like the VBox example, the WheelPane example can now
increase and decrease the number of circles.