Use string constants for layer item type name

GUI/Models/DomainObjectBuilder.cpp

GUI/Models/ItemCatalog.cpp

GUI/Models/LayerItem.cpp

GUI/Models/LayerItem.h

GUI/Models/MaterialItemUtils.cpp

GUI/Models/MultiLayerItem.cpp

GUI/Views/RealSpaceWidgets/RealSpaceBuilder.cpp

GUI/Views/RealSpaceWidgets/TransformTo3D.cpp

GUI/Views/SampleDesigner/SampleViewAligner.cpp

GUI/Views/SampleDesigner/SampleViewFactory.cpp

Tests/UnitTests/GUI/TestMultiLayerItem.cpp

Tests/UnitTests/GUI/TestSessionItemTags.cpp