UI translation: now addons' authors have all needed tools to manage their own translations.

* Update: update (or create) an addon's translation data (as some py code).
* Export PO: export an addon's translation data in a set of pot/po's files.
* Import PO: import a set of po's files to update an addon's translations.

Note: current i18n doc on wiki is completely outdated, I will try to fix this in following days.