qt: add message field to SendCoinsRecipient

Also update URI parsing to fill in this field.
Note that the message is not currently used in any way with the client.
It should be stored with the transaction.

src/qt/walletmodel.h

@@ -21,11 +21,14 @@ QT_END_NAMESPACE
 class SendCoinsRecipient
 {
 public:
-    SendCoinsRecipient() : amount(0) { }
+    explicit SendCoinsRecipient() : amount(0) { }
+    explicit SendCoinsRecipient(const QString &addr, const QString &label, quint64 amount, const QString &message):
+        address(addr), label(label), amount(amount), message(message) {}
 
     QString address;
     QString label;
     qint64 amount;
+    QString message;
 
     // If from a payment request, paymentRequest.IsInitialized() will be true
     PaymentRequestPlus paymentRequest;