make BitcoinAmountField::setReadOnly() usable

- use it for secure payment-requests (this change allows a copy&paste of
  the amount and looks a little nicer than just a disabled GUI element)

src/qt/sendcoinsentry.cpp

@@ -174,6 +174,7 @@ void SendCoinsEntry::setValue(const SendCoinsRecipient &value)
         ui->payTo_s->setText(recipient.authenticatedMerchant);
         ui->memoTextLabel_s->setText(QString::fromStdString(details.memo()));
         ui->payAmount_s->setValue(recipient.amount);
+        ui->payAmount_s->setReadOnly(true);
         setCurrentWidget(ui->SendCoinsSecure);
     }
 }