* All parameters such as content type, field type, selection list are on
  vtkSelectionNode.
* All constants are on vtkSelectionNode instead of vtkSelection.
* The SELECTIONS content type is deprecated.
* To create a simple single-node selection, you must create an
  instance of vtkSelectionNode and an instance of vtkSelection, and add
  the node to the selection. You may also use vtkSelectionSource.
* vtkConvertSelection has a few new useful static functions
  To obtain the list of selected items in a data object.