GTK: fixed bogus "(Unnamed)" strings in the message line

* An empty message line would actually contain "(Unnamed)".
* Info popups must discern the "(Unnamed)" string (e.g. a file with that name)
  from the actual unnamed buffer since when clicking the unnamed buffer, you
  must insert nothing (`EB$`). Therefore, the unnamed buffer is represented
  as an empty string. "(Unnamed)" is just a placeholder for rendering.
  This was carried over into TecoGtkLabel which always rendered the empty string
  as "(Unnamed)".
  But TecoGtkLabels are used for the info and message lines as well.
* Therefore the fallback/placeholder string is now configurable per label.
* On the downside, this wastes one more machine word per TecoGtkLabel.
  The alternative would have been to use {NULL, 0} as the representation
  for unnamed buffers, so you can actually discern the empty string from the
  unnamed buffer representation.
  However it feels wrong to have this kind of info-popup-specific handling
  in a more generic label widget.
  Also, for consistency we'd have to touch the Curses UI as well where
  the unnamed buffer is also currently internally represented by empty
  strings (as opposed to NULL).
* Regression introduced by 0c89fb700957e411885e7e7835e15f441e8b5e84,
  so it was in v2.5.0.

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