Preparing for a stable branch of flatpak

[Alexander Larsson]

The latest version of flatpak (0.6.14) is now in debian unstable, and
the release date for Debian 9 which it is the base for is very soon.
Once it is released the flatpak in it will be around for a long time,
and we will not generally be able to get new features into it, only
bugfixes.

This would be a good time to declare the current version of flatpak at
some kind of stability point in terms of backwards compatibility. In
other words, we should be careful about adding features that cause
flatpak:ed apps to not work on that version. Additionally it would make
sense to have a branch of flatpak based on this that collects only
bugfixes, which could be used by debian (and others) for targeting the
stable release.

So, I'm considering doing a 0.8.0 release soon, which will be such a
branch, and then keep master on 0.9.x as an unstable version which gets
new features.

This begs the question, is there some feature missing in the current
flatpak that would be important to have in the release? I'm primarily
thinking about features that affect installing and running flatpaks
now, not about building or maintaining apps, as developers can always
use a later version to build things, but many users are going to be
stuck with older versions.