Create flatpak version

[thibaultmol]

At this point flathub has shown to be the clear winner when it comes to popularity in linux app packaging that can work on any linux distro.
Most people don't know what to do with an appimage (sorry, but this is the reality. You should still build that version tbc!)
But I think it would make sense to also make it available as a flatpak on flathub and even get you verified as it's creator: https://docs.flathub.org/docs/for-users/verification/