Make tracking SecondaryLoop.enter/exit methods easier

Here is a code snippet from java.awt.SecondaryLoop JavaDoc:

   SecondaryLoop loop;

   JButton jButton = new JButton("Button");
   jButton.addActionListener(new ActionListener() {
       {@code @Override}
       public void actionPerformed(ActionEvent e) {
           Toolkit tk = Toolkit.getDefaultToolkit();
           EventQueue eq = tk.getSystemEventQueue();
           loop = eq.createSecondaryLoop();

           // Spawn a new thread to do the work
           Thread worker = new WorkerThread();
* worker.start();

           // Enter the loop to block the current event
           // handler, but leave UI responsive
** if (!loop.enter()) {
               // Report an error
           }
       }
   });

   class WorkerThread extends Thread {
       {@code @Override}
       public void run() {
           // Perform calculations
           doSomethingUseful();

           // Exit the loop
*** loop.exit();
       }
   }

The code marked with (*) spawns a new thread. Note that (***), which is called on this new thread, can potentially be invoked before the loop is started with (**). This is not a problem in java.awt.Secondary loop, however there's no easy way for developers to detect and resolve this situation.