Sets the current header of the @row.
This is only allowed to be called from a [callback@Gtk.ListBoxUpdateHeaderFunc]. It will replace any existing header in the row, and be shown in front of the row in the listbox.
the header, or %NULL
See Implementation
Sets the current header of the @row.
This is only allowed to be called from a [callback@Gtk.ListBoxUpdateHeaderFunc]. It will replace any existing header in the row, and be shown in front of the row in the listbox.