Anim: name new action slots after the last-used slot

When creating a new action slot for a data-block, via the '+ New' button
of the slot selector, use the name of the slot that was last used by
that data-block. Previously, it would always use the name of the
data-block, interfering with slot auto-selection when toggling between
Actions.

Pull Request: https://projects.blender.org/blender/blender/pulls/138326

scripts/startup/bl_operators/anim.py

@@ -712,7 +712,8 @@ class ANIM_OT_slot_new_for_id(Operator):
         if adt.action_slot:
             slot = adt.action_slot.duplicate()
         else:
-            slot = adt.action.slots.new(animated_id.id_type, animated_id.name)
+            slot_name = adt.last_slot_identifier[2:] or animated_id.name
+            slot = adt.action.slots.new(animated_id.id_type, slot_name)
 
         adt.action_slot = slot
         return {'FINISHED'}