Editor: Avoid SetSceneCommnd when importing file

[ycw]

The issue: Importing 'object' scene(.json) currently performs a mix of 'appending object to default scene' and 'replacing default scene userData', this is problematic because objects in default scene may have scripts consuming userData of default scene, after importing one/multiple scenes(.json), the default scene's userData get replaced by the last imported one, e.g.:

A simple scene with a plane which has a script consuming scene.userData

Camera
Scene  <<< holding original scene userData
   Plane 

After importing scene1.json

Camera
Scene  <<< holding scene1 userData 
   Plane
   Box   <<< object from scene1

After importing scene2.json

Camera
Scene  <<< holding scene2 userData
   Plane
   Box   <<< object from scene1
   Ring  <<< object from scene2

I.e. It is possible that when users PLAY the project, it'll throw because the original scene userData is gone.

---

This PR replaced SetSceneCommand by AddSceneCommand, so each imported file is imported as a child of default scene, the graph for the above mentioned example would become:

Camera
Scene  <<< holding original scene userData
  Plane
- Scene1  <<< holding scene1 userData
    Box 
- Scene2  <<< holding scene2 userData
    Ring

This way, the script of Plane can even access the imported scenes' userData via scene graph traversal

---

If replacing scene is desired, then I suggest implement File>Open Project instead because it has better semantic.

Implemented, see #28351

[Mugen87]

Do you mind rebasing the PR?

[Mugen87]

Do you mind rebasing the PR?

Never mind! I did it now.

[ycw]

thanks