Fix #139195: Preferences are not automatically saved after keyitem_restore

Tag preference dirty when keyitem_restore is called. When window is
closed and is_dirty=true, WM_exit_ex() will call the function to update
the prefs.

Pull Request: https://projects.blender.org/blender/blender/pulls/139254

scripts/startup/bl_operators/userpref.py

@@ -381,6 +381,7 @@ class PREFERENCES_OT_keyitem_restore(Operator):
 
         if (not kmi.is_user_defined) and kmi.is_user_modified:
             km.restore_item_to_default(kmi)
+            context.preferences.is_dirty = True
 
         return {'FINISHED'}