[qmacnativewidget] macnativewidget not draw when events processed before being added to window

1) a qmacnativewidget is created
2) the qapplication processes its events
3) the qmacnativewidget is added to a view

in this case, the widget is not drawn. in my experiments i found a way to use a proxy-class, which implements NSView::viewDidMoveToWindow to hide() and show() the content, but that is not a general solution for us.

note, this issue will only happen if qApp->processEvents() is called before [view addSubview: nativeWidget->nativeView()];