[vtkusers] revamped ImagePlaneWidget tcl example

[dean.inglis at on.aibn.com]

Hi all,

I have checked in a revamped ImagePlaneWidget.tcl 
in /Examples/GUI/Tcl. It has some nifty tcl/tk
GUI elements and makes use of a vtkTkRenderWidget. 
Please bear in mind that my primary development
lang is c++, not tcl, so if someone knows a more
efficient/correct way, help me out. The example
is (hopefully) well documented.

The example demonstrates how to use the 
vtkImagePlaneWidget to probe a 3D image dataset 
with three orthogonal planes.  
Buttons are provided to:
a) capture the render window display to a tiff file
b) x,y,z buttons reset the widget to orthonormal 
positioning, set the horizontal slider to move the
associated widget along its normal, and set the
camera to face the widget  
c) right clicking on x,y,z buttons pops up a menu to set the associated widget's reslice 
interpolation mode.

The image capture makes use of a vtkTiffWriter and vtkWindowToImageFilter pair.  A file called 
"image.tif" is dumped into the directory where the
example resides: you may want to change this 
behaviour.

enjoy,
Dean