Use string constant for BasicAxisItem and AmplitudeAxisItem type name

GUI/Models/AxesItems.cpp

GUI/Models/AxesItems.h

GUI/Models/BeamItems.cpp

GUI/Models/ComponentProxyStrategy.cpp

GUI/Models/GroupInfoCatalog.cpp

GUI/Models/InstrumentItems.cpp

GUI/Models/ItemCatalog.cpp

GUI/Models/SpecularBeamInclinationItem.cpp

GUI/Views/IntensityDataWidgets/ColorMap.cpp

GUI/Views/IntensityDataWidgets/ProjectionsPlot.cpp

GUI/Views/PropertyEditor/ComponentUtils.cpp

GUI/Views/PropertyEditor/ComponentUtils.h

GUI/Views/SpecularDataWidgets/SpecularPlot.cpp

Tests/UnitTests/GUI/TestSavingSpecularData.cpp