qt: move bitcoin URI formatting to guiutil

Follow the same pattern as the parseBitcoinURI function.

src/qt/guiutil.h

@@ -35,6 +35,7 @@ namespace GUIUtil
     // See Bitcoin URI definition discussion here: https://bitcointalk.org/index.php?topic=33490.0
     bool parseBitcoinURI(const QUrl &uri, SendCoinsRecipient *out);
     bool parseBitcoinURI(QString uri, SendCoinsRecipient *out);
+    QString formatBitcoinURI(const SendCoinsRecipient &info);
 
     // Returns true if given address+amount meets "dust" definition
     bool isDust(const QString& address, qint64 amount);