GDK W32: Only use owner windows for popups

GDK W32: Only use owner windows for popups

GTK4 doesn't have WS_CHILD windows anymore, so hWndParent argument
to CreateWindowEx() is always interpreted as the owner window,
not the parent window.

A window with an owner:
* is above the owner in Z-order
* is destroyed when the owner is destroyed
* is hidden when the owner is minimized
This is enforced by the OS.

GTK can only allow this for popup windows.

Desktop window must never[0] be an owner.

[0]: https://devblogs.microsoft.com/oldnewthing/20040224-00/?p=40493