Integration work needed for >=xdg-desktop-portal-1.18.0

Hi all,

xdg-desktop-portal-1.18.0 changes its behaviour if no portals are
configured by the user, installed portals, or desktop environments.

Simon has a good summary at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1050911#35
for what one needs to do.

I've filed a Gentoo tracker bug at https://bugs.gentoo.org/915704
with a list of things I'm aware of that need handling, but I'll
include a list here of some useful references:
* https://github.com/flatpak/xdg-desktop-portal/issues/1017
* https://github.com/flatpak/xdg-desktop-portal/issues/1077
* https://github.com/flatpak/xdg-desktop-portal/issues/1102

Thanks to abby from Void and to psykose as well for the heads up.

best,
sam