1e7e62f8cd
(for new functions...). One might still want to be able to pass down a user-data pointer to be used by the UI. However, ex_data doesn't quite cut it, since that means the appropriate index to it might need to be shared between parts that aren't really related in that sense, and would require the currently hidden (static) index holders to be uncovered. Not a good thing. Therefore, add the possibility to add a user-data pointer to a UI.