Fix Sushi preview window parenting under Wayland

This MR adds support for the org.gnome.NautilusPreviewer2 interface. The (only so far) difference with the v1 interface is that instead of a xid, we now take a window handle using the same convention of Flatpak portals [1].

Together with sushi!7 (merged), which I'll merge when this is accepted, this fixes sushi#1 (closed).

[1] https://flatpak.github.io/xdg-desktop-portal/portal-docs.html#parent_window