Fix #106966: Register undo-step when unlinking data-block

When unlinking IDs like GN tree, materials, etc. undo step is not created.
To fix this, assign `undo_push_label` a string value for registering an
undo-step (with the help of `ED_undo_push`)

Pull Request: https://projects.blender.org/blender/blender/pulls/108452

source/blender/editors/interface/interface_templates.cc

@@ -973,6 +973,9 @@ static void template_id_cb(bContext *C, void *arg_litem, void *arg_event)
         id->us = 0;
         undo_push_label = "Delete Data-Block";
       }
+      else {
+        undo_push_label = "Unlink Data-Block";
+      }
 
       break;
     case UI_ID_FAKE_USER: