Merge #2215: [GUI] Automatically set the lowest possible Custom Fee w…

…hen user provided fee is too low

60bc468 Automatically set the lowest possible Custom Fee when user type in fee that is too low. (MishaPozdnikin)

Pull request description:

  ### What does it do?
  Automatically set the lowest possible Custom Fee when user type in fee that is too low.
  More details: #1836

  Demonstration:

  ![LowestFeeDemonstration](https://user-images.githubusercontent.com/17647788/109294172-83840800-7835-11eb-8f55-bcbedbfe5b73.gif)

ACKs for top commit:
  random-zebra:
    Thanks for the contribution! LGTM. utACK 60bc468
  furszy:
    Code review ACK 60bc468.
  Fuzzbawls:
    ACK 60bc468

Tree-SHA512: 34d1a6c11c841ebf3a9c337183b0087e89c3190507894c1c948ddde4812151b43a658e85d40bfcd1b8ee8dd6ba076f6349c3839f9a1767c2cff463b840cf68e1

src/qt/pivx/sendcustomfeedialog.cpp

@@ -127,6 +127,9 @@ void SendCustomFeeDialog::accept()
         inform(tr("Fee too high. Must be below: %1").arg(
                 BitcoinUnits::formatWithUnit(walletModel->getOptionsModel()->getDisplayUnit(), insaneFee)));
     } else if (customFee < CWallet::GetRequiredFee(1000)) {
+        CAmount nFee;
+        walletModel->getWalletCustomFee(nFee);
+        ui->lineEditCustomFee->setText(BitcoinUnits::format(walletModel->getOptionsModel()->getDisplayUnit(), nFee));
         inform(tr("Fee too low. Must be at least: %1").arg(
                 BitcoinUnits::formatWithUnit(walletModel->getOptionsModel()->getDisplayUnit(), CWallet::GetRequiredFee(1000))));
     } else {