Moved ExternalProperty files to Models

GUI/Models/ExternalProperty.cpp

GUI/Models/ExternalProperty.h

GUI/Models/SessionItemUtils.cpp

GUI/Models/SessionXML.cpp

GUI/Views/MaterialEditor/MaterialEditor.cpp

GUI/Views/MaterialEditor/MaterialItemUtils.h

GUI/Views/PropertyEditor/PropertyEditorFactory.cpp

GUI/Views/RealSpaceWidgets/RealSpaceBuilder.cpp

GUI/Views/RealSpaceWidgets/TransformTo3D.cpp

GUI/Views/SampleDesigner/ILayerView.cpp

Tests/UnitTests/GUI/Comparators.cpp

Tests/UnitTests/GUI/TestExternalProperty.cpp

Tests/UnitTests/GUI/TestLayerItems.cpp

Tests/UnitTests/GUI/TestSessionItemData.cpp

Tests/UnitTests/GUI/TestSessionItemUtils.cpp