Add Varlink to XDG DESKTOP PORTAL

[tgmatos]

Expected Behavior

When dealing with alternative implementations of the Flatpak launcher a second dbus daemon overhead (xdg-dbus-proxy) limits the freedom of implementation. Varlink specs can give more freedom to approach the implementation. This feature has been asked before: https://blogs.gnome.org/mclasen/2019/10/03/some-flatpak-updates/#comment-20194

Current Behavior

Currently Flatpak doesn't have support for Varlink specs.

Anything else we should know?

(Didn't find any flag to ask for this feature, so sorry to do it using a bug report.)

[ryanabx]

Varlink has been adopted by systemd in swaths of their codebase, and it being simpler and introspectable without special tools is a nice bonus. It's also bus-less, which I like.