Add NOTIFICATION_PREDELETE_CLEANUP notification to fix C# Dispose()

[raulsntos]

New notification sent after NOTIFICATION_PREDELETE to let Objects cleanup at the very end, it should be the last notification sent.

• This is one of the options I suggested in C#: Fix NOTIFICATION_PREDELETE does not trigger ExitTree #83262 (comment). I think it's the easiest and safest thing to do and avoids having to revert Fix Object::notification order #78634.

NOTIFICATION_PREDELETE_CLEANUP is used by C# scripts to cleanup (calls Dispose()), instead of NOTIFICATION_PREDELETE. This ensures that it's the last thing we do, after disposing we can't do anything else with the C# instances.

• Fixes C#: Script methods bound to tree_exited / _ExitTree() can't be found. #82279.
• Supersedes C#: Fix NOTIFICATION_PREDELETE does not trigger ExitTree #83262.

[AThousandShips]

This needs to be clearly reserved so future notifications don't use it, especially if we use a system based on registered values to track overlap

[raulsntos]

It seems we use different magnitudes¹ to avoid overlap (e.g.: the Node notifications are 1X, CanvasItem notifications are 3X, Control/Node3D notifications are 4X, Window notifications are 10XX). But I agree it can be error prone.

Footnotes

1. Not sure if magnitude is the right word. ↩

[reduz]

I think this is a good solution and I am all for solving this problem and move forward. I think the following should be considered.

• These are hacks that keep piling up because Mono is not using extension API, otherwise this could be solved entirely within extension API, either with something existing or new.
• As this is basically a temporary hack until we have the time and resources to port Mono to extension API, I suggest not exposing this notification to script (remove the bind constant) and use it internally from Mono, then we remove it when we deprecate the non extension mono sometime in the far future.

EDIT: I apologize, just realized you are not actually binding this, so its fine as is.

[akien-mga]

Discussed with @reduz, seems good as a stopgap measure for the C# predelete, until we can have a more robust system.

Then we would remove that new notification - so PR reviewers should be aware of not letting it be exposed/used more internally, unless there's a very clear use case validated by the core team.

[akien-mga]

Thanks!