Prevent I18N from showing default value or key if a messages file is loaded

reproduce (in some machines at least)

1. on chrome -> network tab -> disable cache
2. go to https://nightly.nuxeo.com/nuxeo/ui/#!/search/default
3. the column header seem to have the key value (for some people)

expected
There should always appear the translation if there are one on messages json

related info

  document.dispatchEvent(new Event('i18n-locale-loaded'));

Is ran before any Nuxeo.I18nBehavior is attached (where we add the listener for the event).

On

 attached: function() {
        this.localeLoadedHandler = this.refresh_18n.bind(this);
        document.addEventListener('i18n-locale-loaded', this.localeLoadedHandler);
      },

Add a condition at the end of attached method to check if i18n locale was already loaded and run this.localeLoadedHandler