[Feature Request] Flatpak support on linux

[that_dark_elf]

I was looking to add flatpak support for Daggerfall unity. For those not familiar see: https://flatpak.org/ and https://flathub.org/home. It is basically a packaging format that works across various linux distributions, has sandboxing features and allows for automatic updates (auto-updates are built into the individual GUI stores; see gnome software center). The end goal of this would to get Daggerfall Unity into flathub, which is currently acting as a central repo for most flatpaks. Currently OpenMW is on there for Morrowind: https://flathub.org/apps/details/org.openmw.OpenMW

I have a *working* manifest which is used to build the flatpak. So far the game launches, however it is a little jankey in that it takes the pre-downloaded zip and extracts it and sets the permissions. Ideally we build from source or at least point it to the url with a sha256sum, I also have to have it extract an icon and set a .desktop file.

This is my first time building a flatpak and most of my programing abilities are in scientific programing (so python, octave and julia). So I am not doing a lot of GUI programing and am not an expert in packaging apps. But if the authors are interested in making this official let me know! There is still a good bit of work that needs to be done.