Bug 791861 - Update appdata.xml AppStream ID to match rDNS-style...

... used for Flatpak.

Additional comments by Jehan after review:
«
This is not only to sync with flatpak. This format is the recommended
format in AppStream spec, even though many historical application still
use the old format.
If ever we decide to propose a dbus service, this may even become
required.
However just changing this is not enough for a proper ID change. I will
make additional change in further commits.
»