Add support for slotted Actions to the NLA evaluation code.
This also affects the pose library code and the Action Constraint. These
both share some Action evaluation logic with the NLA. They now
explicitly looks at only the first Action slot. The Action Constraint will
have to be updated to have an explicit slot selector, but that's for another
commit.
Pull Request: https://projects.blender.org/blender/blender/pulls/127425
Move the functions
* `BKE_pose_apply_action_selected_bones`
* `BKE_pose_apply_action_all_bones`
* and `BKE_pose_apply_action_blend`
out of the blender kernel and into animrig.
This will allow solving #126969 without having to include
animrig code in the blender kernel.
Pull Request: https://projects.blender.org/blender/blender/pulls/126978
