ENH: Improved slice viewer WW/WL and blending

• Blending now supports blending segmentation (not just labelmaps). If no labelmap is selected to be
  shown in the slice viewer then it blends visible segmentation.
• Blending and window level adjustment handles saturation of the values. For example,
  if mouse is moved so much that opacity or window level is saturated and then you move back to the other direction,
  the saturated value starts to get decrease immediately.
• Simplified and removed several unnecessary code from vtkSliceViewInteractorStyle class.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25639 3bd1e089-480b-0410-8dfb-8563597acbee