G2DParentNode handles "undefined" child bounds separately

G2DParentNode handles "undefined" child bounds separately

Previously nodes could only return null to say "I have no bounds".
Now it is possible to return GeometryUtils.undefinedBounds() to say
"ignore my bounds" to G2DParentNode.

Also allow customization of the node class used by ElementPainter to
mark selections under "element nodes" through a new configuration class
ElementPainterConfiguration.

This allows e.g. district to use its own node for this purpose to
optimize the scene graph and customize the bounds it returns.

gitlab #419

Change-Id: I210fd26be5d269847277616b9d5c52ef4f930dd7