generate GNOME flatpak runtimes

Initial version of the flatpak runtimes.

Some notes:

• This uses the flatpak_image plugin from bst-external.
• I've tested Nautilus. It builds and works fine except for gvfs. (that was a stupid mistake, fixed since)
• Some manual tuning may still be needed for what should be sdk-only.
• The following elements have been moved from core/ to sdk/:
  • adwaita-icon-theme.bst
  • glib-networking.bst
  • gnome-themes-extra.bst
  • gsettings-desktop-schemas.bst
  • gvfs.bst
  • yelp.bst
  • (all of the above were in meta-gnome-core-shell.bst except yelp which was in meta-gnome-core-utilities.bst

To build

bst build flatpak-runtimes.bst

mkdir runtimes

for runtime in platform{,-locale} sdk{,-debug,-docs,-locale}; do
  bst checkout flatpak/$runtime.bst runtimes/$runtime
  flatpak build-export --files=files repo runtimes/$runtime next
done

This should produce an ostree repository at repo from which you can install:

flatpak remote-add --if-not-exists --user --no-gpg-verify test-repo repo/

flatpak install --user test-repo runtime/org.gnome.Platform//next
flatpak install --user test-repo runtime/org.gnome.Sdk//next

To build an app with this sdk, simply set the runtime-version to "next".

Closes #16 (closed)