Cleaning up issues with the context-items related to tranfer function editor.

+ Added support for log-mapped color-transfer function. We ensure that when
  using log-scale, the color transfer function widget also enter log-space.

+ vtkPiecewiseControlPointsItem had a bug resulting it deleting of end points
  even when told otherwise. Fixed that.

+ Added new event vtkControlPointsItem::PointsModifiedEvent which if fired when
  control points are changed making it easier for application to do something
  when the points are changed.

+ Improved the usability by adding ability to label control points.

Change-Id: If02bc1139f6ccb3d7c2198b07d10491f902bafda