Allow hiding experimental and deprecated classes from the "Create New Node" and "Create New Resource" dialogs

[theraot]

Describe the project you are working on

Organizing code that has grown organically by being copied from project to project.

Describe the problem or limitation you are having in your project

There are classes that I want to reference from code, but I do not want to give a class_name because they show up in the "Create New Node" dialog. This has led me to reference them via preload, but I've ended up with relative paths which are easy to break as I reorganize my code into folders.

Notably:

• A class that must be instantiated as a child of a Viewport, and there must be only one per Viewport, which will optionally capture all input.
• A class that must be instantiated as a child of a Window, and there must be only one per Window, which will optionally capture the mouse pointer.

These classes will be instantiated from code (the designer should not be adding them to any scene). So they do not have a class_name. And there are multiple places where I check for them, so there are multiple preloads with relative paths for them.

This also means that @abstract (see #4493 (comment), #5641) would not be appropriate for my use case.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Originally I wanted this:

• Annotation to Hide Node with class_name Displaying in the Node Dock #7349
• Create class without showing on node list #1047
• Split class_name responsibility into two keywords #4493

I see this pull request where an annotation is added to allow classes to show up:

• Add top-level @export to separate class_name. godot#63758

However, I agree with this comment godotengine/godot#63758 (comment) because in practice most classes that have a class_name are OK to show up in the dialogs to create nodes or resources. So, I would prefer a negative annotation...

But we already have something similar: We can mark a class as obsolete or experimental. And the dialogs are already aware of them. Thus, I propose to hide them.

Related:

• Add category system for node creating dialog (Superseding #8675) #8741

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

An option either in project settings or a toggle in the "Create New Node" and "Create New Resource" dialogs, which results in the dialog hiding any classes that are marked experimental or deprecated.

Plus we might extend these comment documentation marks to annotate other reasons to not use the class.

If this enhancement will not be used often, can it be worked around with a few lines of script?

I think we might be able to monkey patch the dialogs, however that won't be a few lines of code.

Right now I'm wondering if it is better to:

- Have the instance check if they are the ones created from code, and if they are then disable themselves and put a configuration warning saying to not use them.
- Or figure out how to monkey patch these dialogs to remove my classes.

For my use case it seems simpler to have the instances check if they have an owner, and if so disable themselves and have a configuration warning.

Is there a reason why this should be core and not an add-on in the asset library?

This might be possible via an addon. However I believe this better in core.