Duplicating a strip that references an ID like the scene strip would not
duplicate the scene. This is wanted in some workflows.
To align with the rest of Blender, this changes the behavior for how
strips are duplicated:
* `Shift + D` ("duplicate"): Duplicate the strip and also duplicate the
IDs referenced by the strip. Currently this only affects `Scene`,
`MovieClip`, and `Mask` strips.
* `Alt + D` ("duplicate linked"): Duplicate the strip, but reference the
same IDs. This is the current behavior in `main`.
Part of #144063.
Pull Request: https://projects.blender.org/blender/blender/pulls/144138
6eea75e928