PoC Flatpak packaging

[tinywrkb]

I created an initial Flatpak manifest for Oregano. It's a proof-of-concept and not ready for Flathub submission. Note that I don't plan to submit the app myself, I packaged it just for trying the app.

I'm not sure what the state of the development of the app, but this can be a good distribution method, and also a stable development environment as the specific Flatpak runtime version is not going anywhere.

This was quickly packaged so I might have overlooked some things, like not updating the application id in mimetypes and mime-info, and I haven't done dbus monitoring so I only gave ownership of org.gnome.oregano and not allowed further dbus access.

ngspice and gnucap are packaged in, but I wasn't sure about spice3.

And finally, there's some basic flatpak-builder how-to in the README.md file at the top of my repo.

[drahnr]

Thank you very much for the link! Could you create PR so we can work out a few details? If there is a manifest I would very much like to release it and deprecate the debian packages for which I have no capacity to test anymore.