ENH: Added the enhanced scalarbar actor class based on VTK6.

The current vtkScalarBarActor class allows to display annotations in conjunction with labels. however, in slicer we want to sample the annotations for display just like color values since some color table has more than 10000 values and annotations. This class is based on vtkSlicerScalarBarActor class and only overrides its LayoutTicks method to slightly modify the method to enable sampling the annotations and display it as labels.
Also change colors module logic using the new class.
Also added a python selftest.