New class vtkImageSliceMapper will draw slices of a 3D image, while
new class vtkImageResliceMapper will draw oblique reformatted slices.
New class vtkImageSlice is the corresponding Prop3D for the mappers,
and is a replacement for vtkImageActor, which is now a subclass of it.
New class vtkImageProperty controls window/level, lookup table, etc.

Change-Id: Id6816402404c8d18d1523dec2c36bd809a060170