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");