rm unfinished undo/redo engine (#757)

This resolves #757 (closed).

GUI/View/SampleDesigner/LayerOrientedSampleEditor.h

@@ -47,7 +47,6 @@ private:
     //! Create empty widget with a button for sample creation
     QWidget* createEmptyWidget();
 
-private:
     SampleForm* m_currentSampleWidget;
     QScrollArea* m_scrollArea;
     SampleItem* m_currentSample = nullptr;