Flatpak distribution?

[iaik-jheher]

In the same spirit as Debian, might be a neat distribution vector.

[stefanmore]

Back in the day (for #22), we considered Flatpak, AppImage, and Snap, but decided to go the deb way for now. But afair the package builder supports other package formats as well, so we can check if there is a need (and funding; also for maintenance).