Release as Flatpak

[blacklightpy]

In relation to #338.

I'll also make a request downstream to Void to see if they can patch it for Electron, like they did with Chromium.

[Schneegans]

Hi again! This should be doable. I looked into the Flatpak Builder of Electron. This kinda works, but I think the proper way would be to push this to Flathub and not to distribute homemade flatpaks here on GitHub.

I'll drop a note here as soon as I start working on this. This may take some time as there are other things higher on my priority list, so any contribution would be appreciated here!