qt: Improve GUI responsiveness

QProgressDialog estimates the time the operation will take (based on
time for steps), and only shows itself if that estimate is beyond
minimumDuration. The default minimumDuration value is 4 seconds, and it
could make users think that the GUI is frozen.

src/qt/walletview.cpp

@@ -330,7 +330,6 @@ 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) {