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/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;