Refactoring in ItemFactory and more QString constants.

GUI/coregui/Models/BeamItem.cpp

GUI/coregui/Models/DetectorItems.cpp

GUI/coregui/Models/DomainObjectBuilder.cpp

GUI/coregui/Models/GUIObjectBuilder.cpp

GUI/coregui/Models/IconProvider.cpp

GUI/coregui/Models/InstrumentItem.cpp

GUI/coregui/Models/ItemFactory.cpp

GUI/coregui/Models/ItemFactory.h

GUI/coregui/Models/item_constants.h

...s/Components/InstrumentWidgets/InstrumentEditorWidget.cpp

GUI/coregui/Views/InstrumentView.cpp

GUI/coregui/Views/SampleView.cpp

GUI/coregui/mainwindow/mainwindow.cpp