Allow Quick Open dialog to preview change in scene

[Meorge]

Implements and closes https://github.com/godotengine/godot-proposals/issues/7809.

With this PR, resource references are updated as soon as a new resource is highlighted in the Quick Open dialog. This allows the user to very quickly scroll through resources and preview how they look in the scene.

Video (with music for dramatic effect, of course):

https://github.com/user-attachments/assets/b011895b-bcc9-4795-9a15-6d48b97ee47b