Merge bitcoin-core/gui#343: Improve the GUI responsiveness when progress dialogs are used

4935ac583b qt: Improve GUI responsiveness (Hennadii Stepanov)
75850106ae qt, macos: Fix GUIUtil::PolishProgressDialog bug (Hennadii Stepanov)

Pull request description:

  [`QProgressDialog`](https://doc.qt.io/qt-5/qprogressdialog.html) estimates the time the operation will take (based on time for steps), and only shows itself if that estimate is beyond [`minimumDuration`](https://doc.qt.io/qt-5/qprogressdialog.html#minimumDuration-prop).

  The default `minimumDuration` value is [4 seconds](https://doc.qt.io/qt-5/qprogressdialog.html#details), and it could make users think that the GUI is frozen.

  This PR sets `minimumDuration` to zero for all progress dialogs, that affects ones in the `WalletControllerActivity` class.

ACKs for top commit:
  ryanofsky:
    Code review ACK 4935ac583b. I'm not very familiar with this API but all the changes and explanations make sense and are very clear, and this seems like it should be an improvement.
  promag:
    Code review ACK 4935ac583b.
  jarolrod:
    ACK 4935ac583b

Tree-SHA512: 2ddd74e7fd87894d341d2439dbaa544d031a350f7f57d4c7e9fbba977dc24080fe60fd7a80a542b1647f1de9091d7fd04a36eab695088d4d75fb836548e99b5f

src/qt/walletview.cpp

@@ -331,7 +331,6 @@ void WalletView::showProgress(const QString &title, int nProgress)
         progressDialog = new QProgressDialog(title, tr("Cancel"), 0, 100);
         GUIUtil::PolishProgressDialog(progressDialog);
         progressDialog->setWindowModality(Qt::ApplicationModal);
-        progressDialog->setMinimumDuration(0);
         progressDialog->setAutoClose(false);
         progressDialog->setValue(0);
     } else if (nProgress == 100) {