Make LayerItem::P_THICKNESS private

GUI/Models/DomainObjectBuilder.cpp

GUI/Models/DomainObjectBuilder.h

GUI/Models/GUIDomainSampleVisitor.cpp

GUI/Models/LayerItem.cpp

GUI/Models/LayerItem.h

GUI/Models/MultiLayerItem.cpp

GUI/Models/TransformFromDomain.cpp

GUI/Models/TransformFromDomain.h

GUI/Models/TransformToDomain.cpp

GUI/Models/TransformToDomain.h

GUI/Views/RealSpaceWidgets/RealSpaceBuilder.cpp

GUI/Views/RealSpaceWidgets/RealSpaceBuilder.h

GUI/Views/RealSpaceWidgets/TransformTo3D.cpp

GUI/Views/RealSpaceWidgets/TransformTo3D.h

GUI/Views/SampleDesigner/ILayerView.cpp

Tests/UnitTests/GUI/TestMapperForItem.cpp

Tests/UnitTests/GUI/TestModelUtils.cpp

Tests/UnitTests/GUI/TestMultiLayerItem.cpp