Anim: disable 'New Slot' operator on linked Actions

The 'New Slot' operator now checks for editability of the Action.

Pull Request: https://projects.blender.org/blender/blender/pulls/128708

scripts/startup/bl_operators/anim.py

@@ -691,6 +691,9 @@ class ANIM_OT_slot_new_for_id(Operator):
         if not animated_id.animation_data.action.is_action_layered:
             cls.poll_message_set("Action slots are only supported by layered Actions. Upgrade this Action first")
             return False
+        if not animated_id.animation_data.action.is_editable:
+            cls.poll_message_set("Creating a new Slot is not possible on a linked Action")
+            return False
         return True
 
     def execute(self, context):