community/sway: bring -portalconf up to date with xdg

DesktopNames is now part of the spec, so remove the X- prefix, as gdm only queries DesktopNames. This actually sets XDG_CURRENT_DESKTOP with gdm now.

Also add a warning for users of sway-portalsconf, that not all portals are provided by us. For things like the keyring or color-scheme more portal backends have to be specified.

Also move the sway-portals.conf to /usr/share/... instead of /etc.