Add image representative status text, and add missing English label

Adds a status text showing the status of the image representative. As long as no image file has explicitly been set as representative, a text will be showing: “No file selected.” For some METS/MODS viewers different from Goobi.Presentation, explicitly setting a preview image is required, also if you want it to be the first image.

Also fixes a missing label in the English translation.

This is a supplement to pull request #205.