message: qt, refactor: Allocate SendConfirmationDialog instances on heap
  This change is require for the next commit.
diff: @@ -394,9 +394,10 @@ void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)
  const QString confirmation = model->wallet().privateKeysDisabled() && !model->wallet().hasExternalSigner() ? tr("Confirm transaction proposal") : tr("Confirm send coins");
  const QString confirmButtonText = model->wallet().privateKeysDisabled() && !model->wallet().hasExternalSigner() ? tr("Create Unsigned") : tr("Sign and send");
  - SendConfirmationDialog confirmationDialog(confirmation, question_string, informative_text, detailed_text, SEND_CONFIRM_DELAY, confirmButtonText, this);
  - confirmationDialog.exec();
  - QMessageBox::StandardButton retval = static_cast<QMessageBox::StandardButton>(confirmationDialog.result());
  + auto confirmationDialog = new SendConfirmationDialog(confirmation, question_string, informative_text, detailed_text, SEND_CONFIRM_DELAY, confirmButtonText, this);
  + confirmationDialog->setAttribute(Qt::WA_DeleteOnClose);
  + // TODO: Replace QDialog::exec() with safer QDialog::show().
  + const auto retval = static_cast<QMessageBox::StandardButton>(confirmationDialog->exec());
  if(retval != QMessageBox::Yes)
  {
  