Make sure the printer functions can be called without explicitly openeing the

serial device.

include/interfaces.h

@@ -310,12 +310,12 @@ public:
                                           QVector<int> templateIdx,
                                           QVector<QString> fnames,
                                           QString br,
-                                          QString serial) const = 0;
+                                          QString serial = QString()) const = 0;
 
     virtual bool dc_printTemplate(enum FileTypeJson type,
                                   QVector<int> templateIdx,
                                   QString br,
-                                  QString serial) const = 0;
+                                  QString serial = QString()) const = 0;
 
     virtual void dc_autoRequest(bool on)  const =0;
         // on = true:  select that all READ-Requests are sent automatically