LibGUI: AbstractScrollableWidget GML/JSON

Author: https://github.com/frhun
Commit: 54f459e020
Pull-request: https://github.com/SerenityOS/serenity/pull/10246
Issue: https://github.com/SerenityOS/serenity/issues/5887
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibGUI/ScrollableContainerWidget.h

@@ -27,6 +27,7 @@ protected:
 private:
     void update_widget_size();
     void update_widget_position();
+    virtual bool load_from_json(const JsonObject&, RefPtr<Core::Object> (*unregistered_child_handler)(const String&)) override;
 
     ScrollableContainerWidget();