rename MaterialModel since this is misleading (is no model any more)

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/MultiLayerItem.h

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/MakeItem/ItemWithParticlesCatalog.cpp

GUI/Model/MakeItem/ItemWithParticlesCatalog.h

GUI/Model/Model/MaterialModel.cpp

GUI/Model/Model/MaterialModel.h