message: Add Classnames property `valueSource`
diff: @@ -580,6 +580,7 @@ interface Classnames {
  value?: string; // <input> for the field value
  removeRule?: string; // <button> to remove a Rule
  dragHandle?: string; // <span> as drag-and-drop handle
  + valueSource?: string; // <select> control for value source
  }
  ```
  