listenPreviousLevelButton method

void listenPreviousLevelButton ()

Listens to the 'go to previous level button' click events and decreases the _activeLevel. This method will only be called when the 'messageNoSuchLevel' was shown.

Implementation 

void listenPreviousLevelButton() {
  querySelector("#button_pevious_level").onClick.listen((MouseEvent e) {
    this.setActiveLevel(this._activeLevel - 1);
    this.showLevelOverview();
  });
}