Update flatpaks with atomic

[Firstyear]

Similar to the issue for --all updating docker containers with atomic upgrade, it would be useful if a similar command existed integrating atomic upgrade along with flatpak update. This would be most useful to atomic workstations with system-installed flatpaks.

[giuseppe]

/cc @alexlarsson

[alexlarsson]

Shelling out to flatpak update --system should be enough to make this work. I'm not sure how the atomic update command works though.