Follow-up to #6601

We take Windows Explorer as reference (see #6601 (comment))

Current appearance:

Only columns enabled in the preferences are shown:

How to enable addition of new columns (as shown at the screenshot at #6601 (comment))?

We think "More..." ("Mehr..." in German) is missing:

After a click, a new popup opens

Would be nice if that was added here, too. Would that be possible?

This list should be filled with all fields known to JabRef. In case a user wants to add more fields, they must go to the preferences and add the custom field there.