How to have an Event on setScene : How can I make a custom Event that triggers on `Stage.setScene()`?  
In my code, the button switches the Scenes and that works fine. However I would like to extend the Stage to have an additional Event that is triggered when a button or possibly any other Element triggers a setScene.

Example:
```sql
package sample;

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.stage.Stage;

public class Main extends Application {

    public static void main(String[] args) {
        Application.launch(args);
    }

    @Override
    public void start(Stage stage) {
        Group g1 = new Group();
        Button b1 = new Button("2");
        g1.getChildren().setAll(b1);
        Scene scene1 = new Scene(g1, 50, 50);

        Group g2 = new Group();
        Button b2 = new Button("1");
        g2.getChildren().setAll(b2);
        Scene scene2 = new Scene(g2, 50, 50);

        stage.setScene(scene1);
        stage.setTitle("JavaFX Application Life Cycle");

        b1.setOnAction(actionEvent -> {
            System.out.println("1");
            stage.setScene(scene2);
        });
        b2.setOnAction(actionEvent -> {
            System.out.println("2");
            stage.setScene(scene1);
        });
        stage.show();
    }
}
