[vtkusers] vtkContinuousValueWidget and Representation

[Alex Malyushytskyy]

I came across vtkContinuousValueWidget and tried to use it.
It looks like it and vtkContinuousValueWidgetRepresentation are
incomplete classes  and
both are abstract classes which can't be even instantiated.
At the same tome I have not found any class which is derived from them.

Any reason this code is included and documented?
Are these classes going to be maintained?
If not could they be removed from official version.