reduce SessionItem usage for MultiLayerItem

GUI/Model/From/GUIDomainSampleVisitor.cpp

GUI/Model/From/SampleListModel.cpp

GUI/Model/Job/JobModel.cpp

GUI/Model/Job/JobModelFunctions.cpp

GUI/Model/Sample/MultiLayerItem.cpp

GUI/Model/Sample/MultiLayerItem.h

GUI/Model/To/DomainObjectBuilder.cpp

GUI/View/SampleDesigner/SampleListView.cpp

GUI/View/Toplevel/SimulationView.cpp

Tests/Unit/GUI/TestMapperForItem.cpp

Tests/Unit/GUI/TestModelUtils.cpp

Tests/Unit/GUI/TestMultiLayerItem.cpp

Tests/Unit/GUI/TestParaCrystalItems.cpp

Tests/Unit/GUI/TestSessionModel.cpp