The "new/duplicate" button in the Action Slot selector did not actually
duplicate, and always acted as a "new" button.
This introduces the RNA function `ActionSlot.duplicate()`, which takes
care of duplicating all the animation data associated with the slot as
well. The semantics of this function should remain valid in the
future, when Actions support multiple layers & strips. Note that this
new function does not assign the slot, it just duplicates it and its
data. The assignment of this duplicated slot is done in Python,
through the already-existing API for this.
Pull Request: https://projects.blender.org/blender/blender/pulls/137087
9b1a34e83e