I don't think GTK+ does anything for placeholders. gtkbuilderparser.c has this:

 ...
  else if (strcmp (element_name, "child") == 0)
    parse_child (data, element_name, names, values, error);
 ...
  else if (strcmp (element_name, "placeholder") == 0)
    {
      /* placeholder has no special treatmeant, but it needs an
       * if clause to avoid an error below.
       */
    }

if anything, glade needs to do something here

(In reply to comment #1)
> I don't think GTK+ does anything for placeholders. gtkbuilderparser.c has this:
> 

I have a feeling Christian is referring to placeholder text, as in the
text one would see in a GtkEntry when no text is present.

However, I'm not sure that doing such a feature is good for listbox as is implies some highly specific usages of listbox (i.e. I think it's overspecializing for what should be a basic GTK+ building block).

In the past I've often added an editable row to treeviews with some italic gray text such as " < Enter a foo > " to serve a similar purpose as what I think Christian is suggesting.

As Christian just clarified, this is about the existing function

gtk_list_box_set_placeholder

This functionality is not currently available in GtkBuilder - could be done as
<child type="placeholder">