New abstract DetectorItem base class introduced.

GUI/coregui/Models/ApplicationModels.cpp

GUI/coregui/Models/DetectorItems.cpp

GUI/coregui/Models/DetectorItems.h

GUI/coregui/Models/DomainObjectBuilder.cpp

GUI/coregui/Models/GUIObjectBuilder.cpp

GUI/coregui/Models/InstrumentItem.cpp

GUI/coregui/Models/ItemFactory.cpp

GUI/coregui/Models/ModelPath.cpp

GUI/coregui/Models/item_constants.h

GUI/coregui/Views/InstrumentView.cpp

Tests/UnitTests/GUI/TestMapperCases.h

Tests/UnitTests/GUI/TestSessionModel.h