i18n: update translations

- update POTFILES.in with new source files with translations
- update README with how to pull translations from transifex
- update source pot file with new source translations strings
- update source pot file copyright year
- pull new translations from transifex

Change-Id: I30498c131bc77be008ff54d6ec2b9c85dafc223e
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>