Add window handle lifetime

[prokopyl]

This PR adds a new 'w lifetime on the Window type (from the gui extension), which represents the lifetime of the underlying window handle.

(The previously existing 'a lifetime only tracked the gui_api_type's field's lifetime.)

This change allows to track the underlying window handle lifetime, and therefore allows to implement the new RawWindowHandle APIs safely.

On the flip side, some methods to create a Window from raw native handles have been made unsafe where needed.