Volume rendering display node was set to be hidden at the lowest level of the class hierarchy:
* vtkMRMLCPURayCastVolumeRenderingDisplayNode -> this->SetHideFromEditors(1);
* vtkMRMLVolumeRenderingDisplayNode -> this->SetHideFromEditors(false);
* vtkMRMLDisplayNode -> this->HideFromEditors = 1;
* vtkMRMLNode -> this->HideFromEditors = 0;

This was unnecessarily complex and confusing. Simplified it by removing HideFromEditors flag adjustment in all volume rendering display node base class and all children classes:
* vtkMRMLDisplayNode -> this->HideFromEditors = 1;
* vtkMRMLNode -> this->HideFromEditors = 0;

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24997 3bd1e089-480b-0410-8dfb-8563597acbee