add DEFAULT_TRANSACTION_FEE constant in wallet

- as this is a shared Core/GUI setting, this makes it easier to keep them
  in sync (also no new includes are needed)

src/qt/optionsmodel.cpp

@@ -93,7 +93,7 @@ void OptionsModel::Init()
     // Wallet
 #ifdef ENABLE_WALLET
     if (!settings.contains("nTransactionFee"))
-        settings.setValue("nTransactionFee", 0);
+        settings.setValue("nTransactionFee", DEFAULT_TRANSACTION_FEE);
     nTransactionFee = settings.value("nTransactionFee").toLongLong(); // if -paytxfee is set, this will be overridden later in init.cpp
     if (mapArgs.count("-paytxfee"))
         addOverriddenOption("-paytxfee");

src/wallet.cpp

@@ -6,9 +6,9 @@
 #include "wallet.h"
 
 #include "base58.h"
+#include "checkpoints.h"
 #include "coincontrol.h"
 #include "net.h"
-#include "checkpoints.h"
 
 #include <boost/algorithm/string/replace.hpp>
 #include <openssl/rand.h>
@@ -16,7 +16,7 @@
 using namespace std;
 
 // Settings
-int64_t nTransactionFee = 0;
+int64_t nTransactionFee = DEFAULT_TRANSACTION_FEE;
 bool bSpendZeroConfChange = true;
 
 //////////////////////////////////////////////////////////////////////////////

src/wallet.h

@@ -27,6 +27,8 @@
 extern int64_t nTransactionFee;
 extern bool bSpendZeroConfChange;
 
+// -paytxfee default
+static const int64_t DEFAULT_TRANSACTION_FEE = 0;
 // -paytxfee will warn if called with a higher fee than this amount (in satoshis) per KB
 static const int nHighTransactionFeeWarning = 0.01 * COIN;