Add i18n helpers for the new web ui

Nuxeo stored translated string in property files but we need to make these available to client side applications.
Currently samples rely on either hardcoded strings or on some shared JSON so we can work with a static JSON file for now and plug in a i18n endpoint later.
There are some polymer elements to helps working with strings declaratively but a tag cannot be used everywhere so we would still need to helper/expression to access the translations programmatically.