Mac, Add support for "tear off menus" in main menu bar

Note from development: We can support this when Apple starts to support this.

It would be nice if it was possible to use tear off menus in the menu bar on Mac.

Currently this is only possible when not using the mac menu integration, for example with:

• Setting the environment variable: export QT_MAC_NO_NATIVE_MENUBAR=1
• The menu will have a dotted line that will tear off the menu in its own window.
  #include <QtGui>
  #include <QDebug>

int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QMainWindow w;
QMenu * menu = w.menuBar()->addMenu("menu");
menu->setTearOffEnabled ( true );
for (int a=0; a < 15; a++)

w.show();
return app.exec();
}