Integrates commits c85a455e and 2f9c63b0 from extra/sway.
Copied from c85a455e:
> As of version 1.17, xdg-desktop-portal requires desktops to declare
> which portals should be used. For Sway, this requires a file
> /usr/share/xdg-desktop-portal/sway-portals.conf (or a local user
> config).
>
> A reasonable default is to use GTK for the file picker (as wlr doesn't
> have file picker support) and wlr for screencast/screenshot support.
> Fedora does this similarly. [1]
>
> Upstream is not interested in shipping a configuration file [2] so we
> need to do it downstream.
>
> [1] https://src.fedoraproject.org/rpms/sway/blob/rawhide/f/sway-portals.conf
> [2] https://github.com/swaywm/sway/issues/7773c85a455eb62f9c63b053
