Allow GroupExtension as Source for Views

- DrawViewPart had special case logic for App::Part.
  It now handles DocumentObjectGroup and any other
  GroupExtensions as Source.

src/Mod/TechDraw/App/DrawViewMulti.h

@@ -67,8 +67,6 @@ public:
     virtual void onChanged(const App::Property* prop) override;
     //@}
 
-    virtual TopoDS_Shape getSourceShape(void) const override; 
-
     /// returns the type name of the ViewProvider
     virtual const char* getViewProviderName(void) const override {
         return "TechDrawGui::ViewProviderViewPart";