Create Flatpak application and publish to Flathub

Flatpak is a universal package manager for Linux. We can create a Flatpak manifest and perhaps even publish to the flathub-beta branch if the idea is accepted.

At the moment, Stevenarella isn't conveniently packaged, so having an automatic update mechanism would be really beneficial for Linux users.

I have experience with Flatpak and I can write a manifest so we can get started.