Use string constants for multi layer item type name

GUI/Models/ItemCatalog.cpp

GUI/Models/JobItem.cpp

GUI/Models/JobModelFunctions.cpp

GUI/Models/LayerItem.cpp

GUI/Models/MultiLayerItem.cpp

GUI/Models/MultiLayerItem.h

GUI/Models/SampleValidator.cpp

GUI/Views/RealSpaceWidgets/RealSpaceBuilder.cpp

GUI/Views/SampleDesigner/SampleViewAligner.cpp

GUI/Views/SampleDesigner/SampleViewFactory.cpp

GUI/Views/SimulationWidgets/SimulationDataSelectorWidget.cpp

Tests/UnitTests/GUI/TestModelUtils.cpp