A site that locates itself on the perimeter of a figure.
It implements the setNormal() method to change its position
so that a line drawn through the site at the given normal
will appear to originate from the center of the figure.
Currently, perimeter sites are able to locate themselves
on the perimeter of ellipses, rectangles, and arbitrary polygons. For
any other shape, the site locates itself on the perimeter of the bounding
box for the shape.