Distribute MakeHuman as a Flatpak

[poperigby]

I opened an issue for this before but it was closed without an explanation (#91) so I'm opening a new one.

I think this would be a good idea because it makes it easier to distribute on any OS that supports Flatpak, therefore increasing MakeHuman's availability.

Here's the website for Flatpak: https://flatpak.org/

[bhibb]

I would definitely love to see a flatpak of MakeHuman!

[ssokolow]

If it helps to have an example, here's a set of metadata files for building and packaging LGOGDownloader using flatpak-builder that I put together for a feature request on their issue tracker:

https://gist.github.com/ssokolow/26a2cc98cded622ecae43035ba0a6cfa