make material list referencing more OOD and robust

GUI/Model/Group/SelectionDescriptor.h

GUI/Model/Group/SelectionProperty.h

GUI/Model/Item/BeamAngleItems.cpp

GUI/Model/Item/BeamWavelengthItem.cpp

GUI/Model/Item/ItemWithMaterial.cpp

GUI/Model/Item/ItemWithMaterial.h

GUI/Model/Item/LayerItem.cpp

GUI/Model/Item/LayerItem.h

GUI/Model/Item/MesoCrystalItem.cpp

GUI/Model/Item/MesoCrystalItem.h

GUI/Model/Item/MultiLayerItem.cpp

GUI/Model/Item/ParticleCompositionItem.cpp

GUI/Model/Item/ParticleCompositionItem.h

GUI/Model/Item/ParticleCoreShellItem.cpp

GUI/Model/Item/ParticleCoreShellItem.h

GUI/Model/Item/ParticleItem.cpp

GUI/Model/Item/ParticleItem.h

GUI/Model/Item/ParticleLayoutItem.cpp

GUI/Model/Item/ParticleLayoutItem.h

GUI/Model/Item/RectangularDetectorItem.cpp