Add flatpack release

[diegoiast]

RTFM:
https://docs.flathub.org/docs/for-app-authors/submission

New problem:
I think we need to provide a shell - both inside the flackpack environment, and inside the regular machine. But - we don't support multiple sessions in the terminal plugin.

See also #218

[diegoiast]

Reading/using host tools does not work. Some tips:

--filesystem=host

bool inFlatpak = QFile::exists("/.flatpak-info");


// Native
process.start(program, args);

// Flatpak
process.start("flatpak-spawn",
              {"--host", program} + args);