Action Slots point to the IDs they animate, and after swapping IDs they
also need some swappage (as the Action that first animated `id_a` will
now animate `id_b`, and vice versa).
Instead of doing this in the `id_swap()` function (and requiring
knowledge of how that's supposed to be done), just mark these pointers
as dirty so that they're rebuilt at first use.
This commit contains a little more code than strictly necessary, to add
a function in BKE for this cache invalidation. This avoids having to
have a dependency on the animrig module just for this purpose.
Fixes: #130136
Pull Request: https://projects.blender.org/blender/blender/pulls/131809
9f4a0d230f