Hide the window designer title bar preview when HasCloseButton is unchecked

In the window designer, a window whose TitleBarVisible is checked but whose HasCloseButton is unchecked previews a full title bar, but the running window shows no title bar at all.

Unchecking HasCloseButton removes the entire native title bar on the current windowing stack, not just the close button — this platform behaviour is documented on the Window page, but the designer preview does not reflect it. Users who check only TitleBarVisible therefore see a title bar at design time that never appears at runtime, which reads as TitleBarVisible being broken.

Expected: the designer preview matches the running window — when HasCloseButton is unchecked and TitleBarVisible is checked, the preview should show the window border without a title bar, exactly as the app renders.