Don't listen to nodes that are filtered out by the sort&filter proxy model.
Issue #2642

Example of performance on MacBookPro laptop:
AddData (BrainAtlas2012.mrb) took 22741 msecs -10000msecs
CloseScene () took 11827 msecs  -10000msecs
AddData (BrainAtlas2012.mrb) took 20909 msecs
ModifyNode (vtkMRMLScalarVolumeNode1) took 4 msecs
ModifyNode (vtkMRMLScalarVolumeNode2) took 24 msecs
ModifyNode (vtkMRMLScalarVolumeDisplayNode2) took 22 msecs
ModifyNode (vtkMRMLScalarVolumeNode3) took 15 msecs
ModifyNode (vtkMRMLModelHierarchyNode2) took 10 msecs
ModifyNode (vtkMRMLModelNode4) took 0 msecs
ModifyNode (vtkMRMLModelDisplayNode5) took 11 msecs
ModifyNode (vtkMRMLModelHierarchyNode3) took 10 msecs
ModifyNode (vtkMRMLModelStorageNode1) took 0 msecs
AddNode (vtkMRMLModelNode302) took 20 msecs
Layout (3) took 51 msecs
Layout (2) took 24 msecs
RestoreSceneView (vtkMRMLSceneViewNode15) took 865 msecs  -1000msecs
RestoreSceneView (vtkMRMLSceneViewNode15) took 622 msecs

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21586 3bd1e089-480b-0410-8dfb-8563597acbee