ENH: Added capability to pick a node by Ctrl+Click and set the node to be Selected; A flag AllowNodePicking is added to vtkContourWidget to turn on/off this capability. Also, a SelectedNodes polydata is created inside contour represetation to show these "Selected" nodes; again, this is optional with flag ShowSelectedNodes in vtkContourRepresetation. Finally, there are also some new APIs in both VTK and paraview to make contour widget work in client-server framework.