Add context to generic translatable string

This was suggested in bug 671615 by Jérôme Rapinat.

src/gnome-search/dialog-search.c

@@ -1059,7 +1059,7 @@ type_label_to_new_button(const gchar* type_label)
     else
     {
         PWARN("No translatable new-button label found for search type \"%s\", please add one into dialog-search.c!", type_label);
-        return _("New item");
+        return Q_("Item represents an unknown object type (in the sense of bill, customer, invoice, transaction, split,...)|New item");
     }
 }