[TD]implement view stacking

src/Mod/TechDraw/Gui/ViewProviderDrawingView.h

@@ -52,6 +52,7 @@ public:
     ~ViewProviderDrawingView() override;
 
     App::PropertyBool  KeepLabel;
+    App::PropertyInteger StackOrder;
 
     void attach(App::DocumentObject *) override;
     bool useNewSelectionModel() const override {return false;}
@@ -86,6 +87,12 @@ public:
     Connection connectGuiRepaint;
     Connection connectProgressMessage;
 
+    virtual void stackUp();
+    virtual void stackDown();
+    virtual void stackTop();
+    virtual void stackBottom();
+    virtual int getZ() {return StackOrder.getValue();}
+
 private:
     void multiParentPaint(std::vector<TechDraw::DrawPage*>& pages);
     void singleParentPaint(const TechDraw::DrawView* dv);