[3.x] Implement Scene Unique Nodes

[KoBeWi]

Backport of #60298, including changes from #60511
Introduces small compatibility breakage: % is no longer allowed in Node names.

[Mickeon]

Merging this in 3.x would allow more users to try the feature out and possibly address some design issues that may arise before 4.0 goes beta and locks compability-breaking features. I'm all up for that.

[akien-mga]

Thanks!

[elvisish]

Scene unique names cannot be used from autorun scripts as they are in a different scene in the tree, is this intentional?

[KoBeWi]

$"%Node" is functionally equivalent of owner.find_node("Node"). If your unique node belongs to another scene, you need to fetch the scene first, then node, e.g. $"Scene/%Node" or some_scene.get_node("%Node").

[elvisish]

$"%Node" is functionally equivalent of owner.find_node("Node"). If your unique node belongs to another scene, you need to fetch the scene first, then node, e.g. $"Scene/%Node" or some_scene.get_node("%Node").

This could be difficult if you're not sure the name of the main scene you're looking for the node in, which slightly defeats the purpose of this I guess? I think for global autorun work, groups are still faster and easier to work with.

[KoBeWi]

Well, unique nodes were designed to be used in the same scene. If you want global nodes, groups are indeed better.