Two quality-of-life additions to juggle the `Strip` and `KeyframeStrip`
types:
- New function `Layer.strip_add<KeyframeStrip>()` that returns a
`KeyframeStrip` instead of a `Strip`.
- Implicit conversion operator `operator Strip &()` on `KeyframeStrip`
so that `KeyframeStrip` can be passed to a parameter of type
`Strip &`.
No functional changes.
Pull Request: https://projects.blender.org/blender/blender/pulls/122659
e49963c6a7