Fix the closeEvent of the systray example on OS X

The implementation of the close event handler requires handling the two
close events that are received on OS X when the user quits through the
application menu bar or the Command+Q shortcut. The first is an
spontaneous event, and the second a non-spontaneous one with the window
already closed.

Change-Id: I24e3d3f0de4d631bd2d5616c85ce747f085691e0
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

examples/widgets/desktop/systray/doc/src/systray.qdoc

@@ -173,7 +173,11 @@
 
     We have reimplemented the QWidget::closeEvent() event handler to
     receive widget close events, showing the above message to the
-    users when they are closing the editor window.
+    users when they are closing the editor window. On OS X we need to
+    avoid showing the message and accepting the close event when the
+    user really intends to quit the application, that is, when the
+    user has triggered "Quit" in the menu bar or pressed the Command+Q
+    shortcut.
 
     In addition to the functions and slots discussed above, we have
     also implemented several convenience functions to simplify the

examples/widgets/desktop/systray/window.cpp

@@ -102,6 +102,11 @@ void Window::setVisible(bool visible)
 //! [2]
 void Window::closeEvent(QCloseEvent *event)
 {
+#ifdef Q_OS_OSX
+    if (!event->spontaneous() || !isVisible()) {
+        return;
+    }
+#endif
     if (trayIcon->isVisible()) {
         QMessageBox::information(this, tr("Systray"),
                                  tr("The program will keep running in the "