SelectionProperty: rm get() method, currentItem() is used instead

GUI/Model/Descriptor/SelectionProperty.h

GUI/Model/Device/BeamDistributionItem.cpp

GUI/Model/Device/BeamDistributionItem.h

GUI/Model/Device/BeamItems.cpp

GUI/Model/Device/DetectorItems.cpp

GUI/Model/Device/DetectorItems.h

GUI/Model/Device/InstrumentItems.cpp

GUI/Model/Device/InstrumentItems.h

GUI/Model/Sample/ItemWithParticles.cpp

GUI/Model/Sample/MesocrystalItem.cpp

GUI/Model/Sample/ParticleItem.cpp

GUI/Model/Sample/ParticleLayoutItem.cpp