For an NLA strip to use a slotted Action, it needs to specify which slot
to use in that action. This is now handled by two new properties on the
strip in DNA & RNA: `action_slot_handle` and `action_slot_name`.
These serve the same purpose as their counterparts on the `AnimData`
struct.
Note that this commit does NOT add NLA evaluation support for slotted
Actions. It merely allows assigning them. Evaluation, tweak mode
support, etc. will be implemented in future commits.
Pull Request: https://projects.blender.org/blender/blender/pulls/127359
3417934eff