[i372] GUI: rename methods returning item (Closes #372)

GUI/Model/Data/RealItem.cpp

GUI/Model/Data/RealItem.h

GUI/Model/Device/AxesItems.h

GUI/Model/Device/BeamDistributionItem.h

GUI/Model/Device/BeamItems.cpp

GUI/Model/Device/BeamItems.h

GUI/Model/Device/DetectorItems.cpp

GUI/Model/Device/DetectorItems.h

GUI/Model/Device/InstrumentCollection.cpp

GUI/Model/Device/InstrumentCollection.h

GUI/Model/Device/InstrumentItems.cpp

GUI/Model/Device/InstrumentItems.h

GUI/Model/Device/InstrumentLibrary.cpp

GUI/Model/Device/InstrumentLibrary.h

GUI/Model/Device/InstrumentsEditController.cpp

GUI/Model/Device/InstrumentsEditController.h

GUI/Model/Device/SpecularBeamInclinationItem.cpp

GUI/Model/Device/SpecularBeamInclinationItem.h

GUI/Model/FromCore/ItemizeSample.cpp

GUI/Model/FromCore/ItemizeSimulation.cpp