macOS: Re-apply QWindow visibility when moved between NSWindows

When a QWindow is moved between being a child window and a top level,
it gains a corresponding QNSWindow, which we create as part of
QCocoaWindow::createNSWindow(). NSWindows are not visible by default,
so we need to re-apply the QWindow state to the newly created window,
just like we do for other QWindow properties.

Pick-to: 6.6
Change-Id: I809d12b09eeccea7a4ee4e8444f69db46dbd159e
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/plugins/platforms/cocoa/qnswindow.mm

@@ -225,6 +225,7 @@ NSWindow<QNSWindowProtocol> *qnswindow_cast(NSWindow *window)
     m_platformWindow->setWindowFilePath(window->filePath()); // Also sets window icon
     m_platformWindow->setWindowState(window->windowState());
     m_platformWindow->setOpacity(window->opacity());
+    m_platformWindow->setVisible(window->isVisible());
 }
 
 - (NSString *)description