ENH: VolumeRenderingModuleWidget: Extend API adding a new signal
signals to synch the current MRMLVolumeRendeingDisplayNode from the VolumeRenderingModuleWidget to another ModuleWidgets.
See: https://github.com/Slicer/Slicer/pull/432 https://github.com/Slicer/Slicer/pull/377
P.S.: setting NodeReferenceID (i.e. restoring selected node at Slicer restart) as suggested by lassoan will be addressed later in another PR.