Use string constant for group item type name

GUI/Models/BeamDistributionItem.cpp

GUI/Models/ComponentProxyModel.cpp

GUI/Models/ComponentProxyStrategy.cpp

GUI/Models/ComponentUtils.cpp

GUI/Models/FilterPropertyProxy.cpp

GUI/Models/GroupItem.cpp

GUI/Models/GroupItem.h

GUI/Models/InterferenceFunctionItems.cpp

GUI/Models/ItemCatalog.cpp

GUI/Models/ParameterTreeUtils.cpp

GUI/Models/SessionItem.cpp

GUI/Models/SessionXML.cpp

GUI/Views/MaskWidgets/MaskGraphicsScene.cpp

GUI/Views/PropertyEditor/ComponentFlatView.cpp