Allow to edit objects of SubViewports that have the Own World property enabled

[vitorbalbio]

Describe the project you are working on:
Space Tactical RPG https://twitter.com/ZeroPointGame

Describe the problem or limitation you are having in your project:
Not sure if it's a feature request or a bug but Control Objects in Sub Viewports are not shown in the Editor at all when "Own World" is set make they impossible to edit.

Spatial Objects also suffer with that but at least the Gizmo is shown.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
Ideally when selected a object inside a sub Viewport that is using "Own World" the hole scene should change to this "sub world" allowing edit it as in any normal scene.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
Click in any sub viewport object -> Change to the world of this Sub View port allow it to be edited as normally.

If this enhancement will not be used often, can it be worked around with a few lines of script?:
It's used all the time if you're using sub viewports.

Is there a reason why this should be core and not an add-on in the asset library?:
It's a core feature.

[vitorbalbio]

Some related Bug report.
godotengine/godot#20619