C++ Make sure that the layout is properly updated when the model of a for or if changes

The problem is that the backend is using the `meta_property_listener` to know
if the layout must be updated, but the C++ code would not register a dependency

NOTE about the test: The test doesn't really test that it works because
the test backend don't have a meta_property_listener and apply the layout
inconditionally in sixtyfps_send_mouse_click

api/sixtyfps-cpp/include/sixtyfps.h

@@ -552,6 +552,10 @@ public:
             } else {
                 inner->data.clear();
             }
+        } else {
+            // just do a get() on the model to register dependencies so that, for example, the
+            // layout property tracker becomes dirty.
+            model.get();
         }
     }