message: Add visibleChoicesChangedCallback for widgets
diff: @@ -14,6 +14,7 @@ import { settings } from "./settings";
  * It is a base class for checkbox, dropdown and radiogroup questions.
  */
  export class QuestionSelectBase extends Question {
  + public visibleChoicesChangedCallback: () => void;
  private filteredChoicesValue: Array<ItemValue> = null;
  private conditionChoicesVisibleIfRunner: ConditionRunner;
  private conditionChoicesEnableIfRunner: ConditionRunner;
  @@ -669,6 +670,8 @@ export class QuestionSelectBase extends Question {
  if (this.isLoadingFromJson) return;
  this.updateVisibleChoices();
  this.updateVisibilityBasedOnChoices();
  + if (!!this.visibleChoicesChangedCallback)
  + this.visibleChoicesChangedCallback();
  }
  private updateVisibilityBasedOnChoices() {
  if (this.hideIfChoicesEmpty) {
  