Changed to QAbstractItemModel and implemented drag and drop in QTreeView

GUI/coregui/Models/ItemFactory.cpp

GUI/coregui/Models/ItemFactory.h

GUI/coregui/Models/LayerItem.cpp

GUI/coregui/Models/LayerItem.h

GUI/coregui/Models/MultiLayerItem.cpp

GUI/coregui/Models/MultiLayerItem.h

GUI/coregui/Models/ParameterizedItem.cpp

GUI/coregui/Models/ParameterizedItem.h

GUI/coregui/Models/ParticleDecorationItem.cpp

GUI/coregui/Models/ParticleDecorationItem.h

GUI/coregui/Models/SessionModel.cpp

GUI/coregui/Models/SessionModel.h

GUI/coregui/Views/SampleView.cpp

GUI/coregui/Views/SampleView.h

GUI/coregui/Views/SampleViewComponents.cpp

GUI/coregui/utils/GUIHelpers.h