Use string constant for particle core shell item type name

GUI/Models/GUIDomainSampleVisitor.cpp

GUI/Models/ItemCatalog.cpp

GUI/Models/MaterialItemUtils.cpp

GUI/Models/MesoCrystalItem.cpp

GUI/Models/ParticleCompositionItem.cpp

GUI/Models/ParticleCoreShellItem.cpp

GUI/Models/ParticleCoreShellItem.h

GUI/Models/ParticleItem.cpp

GUI/Models/ParticleLayoutItem.cpp

GUI/Models/SampleValidator.cpp

GUI/Models/SessionItemUtils.cpp

GUI/Models/TransformToDomain.cpp

GUI/Views/RealSpaceWidgets/RealSpaceBuilder.cpp

GUI/Views/RealSpaceWidgets/RealSpaceBuilderUtils.cpp

GUI/Views/SampleDesigner/DesignerScene.cpp

GUI/Views/SampleDesigner/SampleViewFactory.cpp