Adding light widget
This adds a vtkLightWidget and vtkLightRepresentation This modern widget is intended to represent a Light It shows as an always visible sphere on the light position and a line between the light position and the focal point. A cone is show in positional mode with a cone angle. LightPosition, FocalPoint and ConeAngle can be changed via interaction. This also adds a test.
Edited by Mathieu Westphal (Kitware)