Call EditorNode::set_edited_scene() manually instead of via the replacing_by signal

[Rindbee]

Should not change scene_root's child node in EditorNode::set_edited_scene() if replaced later using replace_by. Therefore, it is better to allow the child nodes of scene_root not to be modified automatically.

Supersedes #90479.

Fix #73009.

[AThousandShips]

Is this the only case when replace_by can be called? Or can it be triggered elsewhere?

[Rindbee]

This is for the case of replacing the edited scene root. If you want to replace the original scene root with a new node, you need to manually call EditorNode::set_edited_scene() to set the new node. However, for replace_by, it is best not to modify the parent node of the new and old nodes first to meet certain conditions.

Is this the only case when replace_by can be called? Or can it be triggered elsewhere?

The main purpose of using replace_by is to make the new node inherit some data of the old node. If the new node has already obtained this data, there is no need to use replace_by.

[AThousandShips]

Just as long as there aren't cases where the editor depends on the signal alone that should be fine, but I'm unsure if it doesn't depend on it for other contexts or I feel signals just wouldn't have been used for this it's quite odd

[Rindbee]

What confuses me is that EditorNode::set_edited_scene updates 3 edited_scene_root, and it seems unclear which one should be the main flag.

[Rindbee]

Allow the previous node to be set again as before, as we are not sure which one is used as the flag, we may need to unify the 3 edited_scene_root.

[KoBeWi]

I'm not sure what this condition is trying to do 🤔
If p_auto_add is false, it doesn't matter if p_scene has a parent.

[Rindbee]

This is to improve the condition when adding child:

if (p_scene->get_parent() != scene_root) {
	scene_root->add_child(p_scene, true);
}

add_child() does not allow the adding node already has a parent.

Put it here to prevent scene_root from removing old_edited_scene_root but not adding p_scene. The main purpose is to provide more detailed error messages instead of just relying on the error messages in add_child.

[akien-mga]

Thanks!