User data (context) for event callbacks

I use LittelvGL in mixed C/C++ projects. It would be really great if the event callbacks can have optional user data (void*) like the tasks. This way you can create the callback as a static function inside a class and pass the object pointer (this pointer) to the event so that you can access the class object inside the callback.

This should already be doable. Event handlers are passed the object (lv_obj_t *) and you can store user data on that with lv_obj_set_user_data.