BUG: fix error on start up from SurfaceToolbox

Add in import string to fix start up error when 
trying to load the SurfaceToolbox scriptable module:

Traceback (most recent call last):
  File "/projects/birn/nicole/Slicer42/Slicer4-SuperBuild-Debug/Slicer-build/lib/Slicer-4.3/qt-scripted-modules/SurfaceToolbox.py", line 11, in __init__
    parent.helpText = string.Template("""
NameError: global name 'string' is not defined



git-svn-id: http://svn.slicer.org/Slicer4/trunk@22433 3bd1e089-480b-0410-8dfb-8563597acbee