Allow extracting the Surface used to construct a Window.

[nicolasavru]

Wayland allows reusing a surface for the same role it had previously.

[nicolasavru]

I don't love the unwraps, so if you think of a better way to do this, please let me know.

[Ciantic]

I think this is required for "windowless" apps, such as those running as tray icon, and showing window or layer shell only briefly.