[vtkusers] The real difference between QVTKWidget and QVTKWidget2

[Utkarsh Ayachit]

To add to what Marcus said, ParaView wants to switch over to using Qt
5.6 officially and when that happens, we will be using QOpenGLWidget
(through QVTKWidget2). So testing QVTKWidget2 thoroughly in on the
timeline for the May/June.

Utkarsh