Internationalisation (i18n) of the CMS.

Every string should be looked up from a translation document that is stored in the repository, and is editable with the CMS.
In the future, it should be possible to login as a "CMS Translator" user, which will display a little edit icon next to every string shown in the UI. Clicking on this icon will popup a little string editor where the translation can be added. This is very similar to the way we want to do context help (CMS7-1421)