Axis constraint on vtkFinitePlaneWidget

Hi Developers

The vtkFinitePlaneWidget works great, but I do miss a small feature that can be easily added.

Currently the plane can be pushed along the axis and the two points can be moved freely to create a Parallelogram. It is trivial to add a small option that will enforce the plane to be rectangular, e.g. a vtkBool named RectangularShape.

The widget is a little nasty in the sense that there is no abstract interface for its representation, so changing this outside VTK is a clone and own. I do think though that this small addition will be of use.

When RectangularShape is on, moving the two points will simply change the width and height of the widget instead of creating a parallelogram.

I will make a small pull request.