Assigning an Action slot changes two RNA properties on the `AnimData` struct:
- `action_slot`: pointer property used for the selector in the UI, and
- `action_slot_handle`: int property that's the ground truth of the assigned slot.
With this commit, there will only be an override operation stored for the
`action_slot_handle` property, and the pointer property will be ignored.
This also cleans up some other library overrides related things:
- Disallow overriding slot properties (like the slot name).
- Properly handle slot assignment in the case where the library did not have an `AnimData` at the time of the creation of the override, but when it does when applying the override (in `rna_AnimaData_override_apply()`).
Fixes#125978
Pull Request: https://projects.blender.org/blender/blender/pulls/126828
841669c79e