Add filter format support to translation layer; translate header, footer, empty text

At least the header, footer and empty texts support filter formats.

To avoid possible (code) injection by translaters - they might contain php code - they don't trigger any translateability.

This is the reason why i18n added filter format support to the translation API.
To support those elements for clean translation, views could add filter format support for the translation layer on a per-key basis.

The filter based elements would then need to be additionally announced and translation layer called.