Don't allow adding Objects to the project settings

[Calinou]

Godot doesn't support serializing objects.

Note: I haven't tested this since I can't open the Project Settings in the current master branch due to a DisplayServer bug (even in single window mode).

This closes #33667.

[akien-mga]

This fails building and should be rebased (and tested).

[KiritoAM]

@Calinou I've tested this on latest and all seems good.
Regarding #33667, I can't repro this issue on latest without your fix - I can select the Object type from the dropdown and try to create a setting, but it fails to create it (and add it to project.godot upon saving the settings). Not sure exactly where it fails to add the setting; it makes it through ProjectSettingsEditor::_add_setting, but the Object value created in the function is nil.
Would be nice to get your change in so avoid this situation.

[Calinou]

Rebased and tested, it works fine to the extent I can test it. (I still can't select options in dropdown menus due to a DisplayServer regression.)

[KiritoAM]

The IOS build failure doesn't look like it has anything to do with your change. Guessing the procedure in this situation is to just restart the build and hope the build machine doesn't trip over again?

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.2.4.