Turn format methods into element properties

Right now, the format methods in Nuxeo.FormatBehavior that rely on label translation are not recomputed when the locale changes, unless the page is reloaded. The current workaround is to always pass the i18n method as a dependency when calling the format method (ex.: formatDocType(type, i18n) ), but this is verbose and hard to maintain. We can turn these into properties of type Function, which will be recomputed whenever the i18n function changes, either with an observer or by using computed properties.