Fix transaction fee in uBTC

Step for buttons 'up' and 'down' - 0.001. With BTC and mBTC all ok, but
0.001 uBTC is lower than minimal value (satoshi)
User should press 10 times on 'up' button to get 0.01 uBTC

src/qt/bitcoinamountfield.cpp

@@ -145,6 +145,11 @@ void BitcoinAmountField::unitChanged(int idx)
     amount->setDecimals(BitcoinUnits::decimals(currentUnit));
     amount->setMaximum(qPow(10, BitcoinUnits::amountDigits(currentUnit)) - qPow(10, -amount->decimals()));
 
+    if(currentUnit == BitcoinUnits::uBTC)
+        amount->setSingleStep(0.01);
+    else
+        amount->setSingleStep(0.001);
+
     if(valid)
     {
         // If value was valid, re-place it in the widget with the new unit