Merge commit 'ff5e3f15a47f165005e691a88cbe1fa44a84fe0d'

Use a defualt parameter for printer functions to decide if serial device has to be
opened or not.

DCPlugin/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