onDestroyEvent() {
		// execute onDestroyEvent function if given through the constructor
		if (typeof this.settings.onDestroyEvent === "function") {
			this.settings.onDestroyEvent.apply(this, arguments);
		}
	}
onDestroyEvent function<br>
called by the state manager before switching to another state
@name onDestroyEvent
@memberof Stage