rename GUI/Session -> GUI/Items

GUI/CommonWidgets/DesignerHelper.cpp

GUI/CommonWidgets/ItemSelectorWidget.cpp

GUI/CommonWidgets/ItemStackWidget.cpp

GUI/CommonWidgets/ModelTreeView.cpp

GUI/CommonWidgets/SessionItemController.cpp

GUI/DataLoaders/AutomaticDataLoader1D.cpp

GUI/DataLoaders/AutomaticDataLoader1DResultModel.cpp

GUI/DataLoaders/QREDataLoader.cpp

GUI/Items/AxesItems.cpp

GUI/Items/AxesItems.h

GUI/Items/BackgroundItems.cpp

GUI/Items/BackgroundItems.h

GUI/Items/BeamAngleItems.cpp

GUI/Items/BeamAngleItems.h

GUI/Items/BeamDistributionItem.cpp

GUI/Items/BeamDistributionItem.h

GUI/Items/BeamItems.cpp

GUI/Items/BeamItems.h

GUI/Items/BeamWavelengthItem.cpp

GUI/Items/BeamWavelengthItem.h