[wallet] Refactor to use new MIN_CHANGE

* Introduce new constant MIN_CHANGE and use it instead of the
hardcoded "CENT"
* Add test case for MIN_CHANGE
* Introduce new constant for -mintxfee default:
  DEFAULT_TRANSACTION_MINFEE = 1000

src/qt/coincontroldialog.cpp

@@ -567,7 +567,7 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
                 nChange -= nPayFee;
 
             // Never create dust outputs; if we would, just add the dust to the fee.
-            if (nChange > 0 && nChange < CENT)
+            if (nChange > 0 && nChange < MIN_CHANGE)
             {
                 CTxOut txout(nChange, (CScript)std::vector<unsigned char>(24, 0));
                 if (txout.IsDust(::minRelayTxFee))