flatpak: Add flatpak manifests for stable and unstable branches

Drop the old xdg-app based build rules for building a bundle file, in
favour of the new flatpak manifests which can be used to generate a
repository with the command:
   flatpak-builder --repo=repo build-dir flatpak/org.gnome.Hitori-stable.json

This repository can then be turned into a bundle using the instructions
at
http://docs.flatpak.org/en/latest/distributing-applications.html#single-file-bundles.

The use of manifest files means it should now be possible for Hitori to
be built nightly:

https://wiki.gnome.org/Projects/SandboxedApps/Building



Signed-off-by: Philip Withnall <withnall@endlessm.com>