Select newly added nodes on drag and drop in 3D viewport

[viksl]

Implements this proposal: godotengine/godot-proposals#8282 for 3D viewport.

Testing:
Create couple scenes of different types which can be added to a 3D scene and save them as separate. Try to drag and drop these directly in to the viewport (not the scene tree tab that's a different thing and works ok), one by one, multiple files, have some root node selected or not.
What you should see with this change is that the newly added node(s) will be selected immediately so you should be able to work with them in the viewport.
Test Undo and Redo too please.

Huge thanks to KoBeWi for advice.

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Related 2D PR: #84356

[Calinou]

Tested locally, it mostly works as expected.

One issue is that undoing then redoing won't reselect the node that was added through drag-and-drop (unlike when you add a node through other means).

[viksl]

Yeah that's what I thought, it's just that currently I don't really know how to work with the undo-redo system, I assume it's fairly complicated as undos in other programs?

EDIT: Actually I think I have it already working, I'll amend it later on so you can test it in case I'm messing something up.

[viksl]

@Calinou I've updated both PRs, should work with undo/redo now. Would you mind checking it out. I'll add info to 2D PR since there's one strange thing I don't know how to work around but doesn't seem to be an issue which would break functionality as far as I can tell right now.

[akien-mga]

Thanks!