Desktop applications can enumerate their open windows

Desktop applications need a built-in way to enumerate their currently open windows. Add DesktopApplication.Windows() As Array(Of Window) so applications can inspect window state, count open windows, and perform operations such as closing every window except the current one without maintaining a separate application-level array.

The method returns a fresh snapshot array. The array can be changed by the caller without changing application state, while each contained Window remains the live window instance.