Cocoa: Close windows with QWSI::handleCloseEvent()

Calling QWindow::close() directly bypasses some of
the window close logic in the QWindow subclasses.

Change-Id: I208db5600e6a756e25e207eaaf55dcfad255f406
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

src/plugins/platforms/cocoa/qcocoaapplicationdelegate.mm

@@ -223,8 +223,9 @@ static void cleanupCocoaApplicationDelegate()
             // events while the event loop is still running.
             const QWindowList topLevels = QGuiApplication::topLevelWindows();
             for (int i = 0; i < topLevels.size(); ++i) {
-                topLevels.at(i)->close();
+                QWindowSystemInterface::handleCloseEvent(topLevels.at(i));
             }
+            QWindowSystemInterface::flushWindowSystemEvents();
 
             QGuiApplication::exit(0);
             startedQuit = false;