[REG 5.5.1->5.6.2] macOS: Submenus cannot be disabled

Setting submenu disabled with QMenu::setEnabled(false) has no effect. The submenu is still enabled. This can be tested with Qt Example Application by adding
QMenu* subMenu = fileMenu->addMenu(QString("SubMenu"));
subMenu->setEnabled(false);
to menu creation (MainWindow::createMenus() or MainWindow::createActions() in the example).

The problem is not reproducible in Qt 5.5.1 or in Qt 5.4.0.