Fix quitting on macOS

The QuitHandler that proxies our Exit action needs to call
the performQuit() method on the QuitResponse that it receives.
See the Javadoc for com.apple.eawt.QuitHandler.handleQuitRequestWith()
which says:

  /**
   * Invoked when the application is asked to quit.
   *
   * Implementors must call either {@link QuitResponse#cancelQuit()}, {@link QuitResponse#performQuit()}, or ensure the application terminates.
   * The process (or log-out) requesting this app to quit will be blocked until the {@link QuitResponse} is handled.
   ...

Fixes #247

src/main/java/com/tagtraum/perf/gcviewer/ctrl/action/Exit.java

@@ -39,7 +39,8 @@ public void actionPerformed(final ActionEvent e) {
     }
 
     // Used by OS X adaptations
-    public void quit() {
+    public boolean quit() {
         actionPerformed(null);
+        return true;
     }
 }

src/main/java/com/tagtraum/perf/gcviewer/view/util/OSXSupport.java

@@ -112,7 +112,7 @@ public static void addOSXHandler(String handlerClassName,
 
             Class<?> handlerClass = Class.forName(handlerClassName);
             if (action != null) {
-                Object aboutHandlerProxy =
+                Object handlerProxy =
                     Proxy.newProxyInstance(OSXSupport.class.getClassLoader(),
                                            new Class[]{handlerClass},
                                            new InvocationHandler() {
@@ -121,10 +121,14 @@ public Object invoke(Object o, Method method, Object[] args) throws Throwable {
                                                    if (method.getName().equals(handlerMethodName)) {
                                                        action.actionPerformed(null);
                                                    }
+                                                   if (method.getName().equals("handleQuitRequestWith")) {
+                                                       Object quitResponse = args[1];
+                                                       Class.forName("com.apple.eawt.QuitResponse").getDeclaredMethod("performQuit").invoke(quitResponse);
+                                                   }
                                                    return null;
                                                }
                                            });
-                application.getClass().getMethod(handlerSetterMethodName, handlerClass).invoke(application, aboutHandlerProxy);
+                application.getClass().getMethod(handlerSetterMethodName, handlerClass).invoke(application, handlerProxy);
             } else {
                 application.getClass().getMethod(handlerSetterMethodName, handlerClass).invoke(application, (Object) null);
             }