Split the versioning of legacy Actions to slotted ones into two steps:
1. Versioning the Actions themselves, in the regular versioning code
(`do_versions_after_linking_400`). This has to happen in the 'after
linking' stage, and not in the 'before linking' stage, as there's
older 'after linking' code that will break when it gets fed slotted
actions. Any ID that is using a legacy Action will get tagged.
2. Versioning Action assignments, where the correct Action Slot has to
be chosen & assigned for each tagged ID. This has to happen in the
`do_versions_after_setup` stage, as choosing a slot requires that the
actions (both local & linked) have been converted already.
This also includes some necessary changes to the pre-2.50 Action versioning
code.
Note that this change does not handle library overrides. That's dealt
with in !131426.
Pull Request: https://projects.blender.org/blender/blender/pulls/131627
446f0a806e