This patch implements i18n support in the chatview using Jed.

+ gettext style API, transparent for translators.
+ extract string using xgettext, translate po as usual.
+ convert po files to JSON using po2json.
  This has to be done once, by the Jenkins automatic i18n bump for
  instance. The build-chatview-locales script can be used to
  automate the .po -> .json convertion process.
+ po2json generated files are shipped as gresource and loaded into the
  chatview by webkitchatcontainer.

Gitlab: #900
Change-Id: Iaf925e2fd37174fff0b6139fc0019bda91938ace
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>