Adds scene assets (including the catalogue tree) to the `Add` > `Scene` menu.
When selecting a scene asset, the asset import setting is used:
* Link: Scene is linked (cannot be edited, but will be used by the strip)
* Append: Appends the full scene including all IDs to the file.
* Append/Reuse: Appends the scene but will reuse IDs (like mesh data, materials, etc.).
Objects, collections, and the scene itself are not reused.
Part of #144063.
Pull Request: https://projects.blender.org/blender/blender/pulls/141213
2f852d4f00