Unexpose DisplayServer::window_set_window_event_callback

[Sauermann]

Overwriting this function led to problematic behavior, that is impossible to reimplement from GDScript.
It is possible to listen to the events, by overwriting Window._notification, so an alternative to this exposed function is available.

This PR is based on this discussion in RC and on an overview of potential DisplayServer function interferences by bruvzg.

resolve #81830

If additional DisplayServer functions should be unexposed, let me known and i will adjust this PR.

[Riteo]

I 100% support this. It looks like this was added in 8e6960a without further explanation (CC @reduz, hope I'm not bothering) and I'm not sure either what's the usecase for this too.

Just one thing before I approve, do we allow removing API like this instead of making stub deprecated methods?

[akien-mga]

So I asked @reduz about this:

Akien @reduz Can you comment on why you exposed window_set_*_callback methods in DisplayServer? See #84638
reduz @Akien I have no idea why I exposed it, my suggestion is that all functions in DisplayServer for window management are discouraged to use in the docs

Since we identified a number of methods which interfere with Window and should only be used by someone who really knows what they're doing, I think it might indeed be best to put a clear disclaimer in the docs for each of those, but keep them exposed so we don't break compatibility.

#81830 shows that some users were actually using this one at least.

[akien-mga]

Documentation only alternative, for discussion: #84669

My understanding is that these methods could be used to create and manage windows (but not Window nodes!) directly on the DisplayServer. Like RenderingServer can be used to create canvas items (and not CanvasItem nodes), etc.

So using these methods on an existing Window node's ID is risky, but they can be used for something more low level.

[Sauermann]

#81830 shows that some users were actually using this one at least.

The user of that issue was happy with the workaround (using _notification), so I wouldn't count them as "actually using" this functionality.

[akien-mga]

Superseded by #84669.