Use string constant for particle layout item type name

GUI/Models/DomainObjectBuilder.cpp

GUI/Models/ItemCatalog.cpp

GUI/Models/LayerItem.cpp

GUI/Models/MaterialItemUtils.cpp

GUI/Models/ParticleItem.cpp

GUI/Models/ParticleLayoutItem.cpp

GUI/Models/ParticleLayoutItem.h

GUI/Models/SampleValidator.cpp

GUI/Views/RealSpaceWidgets/RealSpaceBuilder.cpp

GUI/Views/SampleDesigner/DesignerScene.cpp

GUI/Views/SampleDesigner/SampleViewFactory.cpp

Tests/UnitTests/GUI/TestParticleLayoutItem.h