inheritence to allow handler to also be used as node callbacks.
Fix to UpdateVisitor to make the visitation of Drawable more consistent
with the way that nodes are traversed.
osgGA (Gui Abstraction). This may break users code, but all it should
require to fix the builds should be a change from
osgUtil::CameraManipulator (etc) to osgGA::CameraManipulator and
include <osgUtil/CameraManipulator (etc) to osgGA/CameraManipulator and
the extra dependency of the link line.