Sandboxed Apps https://wiki.gnome.org/Projects/SandboxedApps from Alex Larsson.
The code is hosted at https://github.com/alexlarsson/xdg-app .
The first xdg-app release https://blogs.gnome.org/alexl/2015/03/31/official-gnome-sdk-... has some more details and summary, and links to the proof of concept builds.