Gui: fix ViewProviderGeometryObject

* fix several linter warnings
* remove code that cannot be executed: inside the constructor it cannot ever happen that getObject()
  returns a valid object
* in ViewProviderGeometryObject::handleChangedPropertyName call the method of the direct base class
  as otherwise this may break the mechanism in the future
* Shape is a property of an extension module -> move its handling to ViewProviderPartExt

src/Mod/Part/Gui/ViewProviderExt.cpp

@@ -844,6 +844,11 @@ void ViewProviderPartExt::updateData(const App::Property* prop)
             }
         }
     }
+    if (propName && strcmp(propName, "Shape") == 0) {
+        // Reapply the appearance
+        const App::Material& Mat = ShapeAppearance[0];
+        setSoMaterial(Mat);
+    }
     Gui::ViewProviderGeometryObject::updateData(prop);
 }