+ * @is_reply: whether the derived subject is for a reply or a forward message
+ *
+ * create a 'derived' subject line for eg. replies and forwards. Note
+ * that this function will use the localized versions of "Re" and
+ * "Fw", unless one of these two versions was already included. For
+ * example replying to an email in Finish would work as:
+ *
+ * "some subject" -> "VS: some subject"
+ * "VS: some subject" -> "VS: some subject"
+ * "Re: some subject" -> "Re: some subject"
+ * "Fw: some subject" -> "VS: Fw: some subject"