No need to construct a Point2D here.
Different, type-based approach to node-shapes

Instead of a single class for all nodes, this adds shapes
for several easy geometric nodes: JBox2D already has circle and polygon
shape implementations, so we only need to initialize these
with the data from the JavaFX node, and update them when needed.

Some other shapes (like most control elements, buttons, etc.) might be
reasonably approximated by rectangles in the future.