QMdiSubWindow: Hide "system" menu icons on macOS

Not only they should not appear (icons in menus is a special
thing on macOS), they don't match the look on the window frame
buttons. So, it's better to hide them completely.

Change-Id: I8814ed3e128480abff4f53fdd40a0bb13f1796ef
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/widgets/widgets/qmdisubwindow.cpp

@@ -1986,6 +1986,13 @@ void QMdiSubWindowPrivate::updateActions()
     for (int i = 0; i < NumWindowStateActions; ++i)
         setVisible(WindowStateAction(i), false);
 
+#ifdef Q_OS_MACOS
+    if (q_func()->style()->inherits("QMacStyle"))
+        for (int i = 0; i < NumWindowStateActions; ++i)
+            if (QAction *action = actions[i])
+                action->setIconVisibleInMenu(false);
+#endif
+
     if (windowFlags & Qt::FramelessWindowHint)
         return;