add the option to choose between usd,eur,btc as currency for mainwindow Pricecheck

src/mainwindow.h

@@ -78,7 +78,9 @@ public:
 
 public slots:
     void slot_change_theme(const QString& themeName);
-
+    void slot_change_currency(const QString& currencyName);
+    
+    
 private:    
     void closeEvent(QCloseEvent* event);