Fix #136947: Duplicate Action Slot doesn't duplicate animation data

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

scripts/startup/bl_operators/anim.py

@@ -679,6 +679,10 @@ class ANIM_OT_slot_new_for_id(Operator):
     Note that _which_ ID should get this slot must be set in the 'animated_id' context pointer, using:
 
     >>> layout.context_pointer_set("animated_id", animated_id)
+
+    When the ID already has a slot assigned, the newly-created slot will be
+    named after it (ensuring uniqueness with a numerical suffix) and any
+    animation data of the assigned slot will be duplicated for the new slot.
     """
     bl_idname = "anim.slot_new_for_id"
     bl_label = "New Slot"
@@ -703,10 +707,14 @@ class ANIM_OT_slot_new_for_id(Operator):
 
     def execute(self, context):
         animated_id = context.animated_id
+        adt = animated_id.animation_data
 
-        action = animated_id.animation_data.action
-        slot = action.slots.new(animated_id.id_type, animated_id.name)
-        animated_id.animation_data.action_slot = slot
+        if adt.action_slot:
+            slot = adt.action_slot.duplicate()
+        else:
+            slot = adt.action.slots.new(animated_id.id_type, animated_id.name)
+
+        adt.action_slot = slot
         return {'FINISHED'}