Add Flatpak support

[sandboiii]

Flatpak has recently gained popularity, this application needs to add flatpak support. It would ensure users would have automatically updated applications on any distro.

Electron currently can't make flatpak bundles, see this issue https://github.com/electron-userland/electron-builder/issues/512 But when it would be able to do it, I suggest to create the flatpak bundle

So, consider this issue as a backlog task

[dentad]

Yes, please provide a flatpak alongside your snap. :-D