Support Flatpak

[ghost]

Please add support for Flatpak. This would make it distro agnostic, resolving #95, and would provide a sandbox. From what I gather packaging for Flatpak is fairly simple, although I have never done it - see https://docs.flatpak.org/en/latest/first-build.html.

[Eric1212]

Flatpak are clearly the way to distribute now in linux, 3.2 should be distributed on flatpak for linux if it's happen. It's depend on @therealglazou i suppose.