Gtk4/Win32: Fix popup focus stealing when CSD is disabled

Prevents Popovers/Menus/etc from stealing focus from the parent window when using Server Side Decorations on Windows.

This fix stops the popup window from being activated (in the Win32 sense of the word), while still keeping it in the foreground and sensitive to user input when the surface is raised.

Before: PkhW4aZHYj VfgFf64qQJ

After: lhf7HPr15N pNoNzTJ2En

I haven't opened an issue for this, as it is minor and I've already found a fix, but please let me know if you'd prefer me to do that in future.