rm unneeded forward declarations, especially of SessionItem

GUI/Model/Descriptor/UIntProperty.h

GUI/View/Import/RealDataSelectorWidget.cpp

GUI/View/Import/RealDataSelectorWidget.h

GUI/View/Instrument/DistributionPlot.cpp

GUI/View/Instrument/DistributionPlot.h

GUI/View/Mask/IShape2DView.cpp

GUI/View/Mask/IShape2DView.h

GUI/View/Plot2D/IntensityDataWidget.h

GUI/View/SampleDesigner/LayerOrientedSampleEditor.cpp

GUI/View/SampleDesigner/LayerOrientedSampleEditor.h

GUI/View/SampleDesigner/SampleEditorController.h

GUI/View/SampleDesigner/SampleView.h

GUI/View/Toplevel/SessionDecorationModel.h