BUG: vtkMRMLSceneViewNode destructor should clear the internal scene...

and not clear the nodes.
Some nodes (e.g. vtkMRMLDisplayableNode) require to be removed from the
scene correclty in order to be correctly destructed. Indeed,
vtkMRMLDisplayableNode::~vtkMRMLDisplayableNode removes all display node
IDs which require to call on the scene.
RemoveReferencedNodeID() can only work if the nodes have been correctly
cleaned up.
This change has revealed some issues in the scene view node scene
references. A failing test has been created: references().
The problem is that vtkMRMLSceneViewNode doesn't update its references at
the right time. And when it does (UpdateScene), the references are wrong.
See 0001995 for more details.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@20034 3bd1e089-480b-0410-8dfb-8563597acbee