Add field focus to open ui string field

When a field in the visual editor gets focus its field label (and optional hint icon) turns blue. This is not the case for the open ui string field yet. The iframe and/or its elements in it get focus, but the parent document has no clue about that.

Can we add new methods to the API to let the extension signal blur and focus events?

Not that the focus-after-tab works differently in chrome and firefox. In FF the iframe element itself seems to get focus first, and you need to hit the tab key another time to move the focus on the first field inside the iframe. In chrome you tab directly into the first field in the iframe.