Flatpak package for Linux

Would be great to offer/build Linux installer on a [Flatpak package](https://flatpak.org/).

Afterward could be [offered on Flathub](https://github.com/flathub/flathub/wiki/App-Submission), so the installation process end up being just `flatpak install flathub org.peerviewer` (`flatpak install peerviewer` will probably work too).