ENH: Use ITK FilterWatcher and rename DifferenceImageFilter2

"itktubeFilterWatcher" is identical to <itkFilterWatcher.h> so use the ITK
version. itkDifferenceImageFilter2 used to clash with itkDifferenceImageFilter,
but now does not with new naming convention and namespace, so rename.

Change-Id: I17743b3f0f34bf000cbace078afc1b62b61c2d8d