Qt: Show dialog on startup for settings version mismatch

For the QMessageBox to appear, error reporting is performed again after
main window creation.

src/duckstation-qt/mainwindow.cpp

@@ -1088,6 +1088,13 @@ void MainWindow::startupUpdateCheck()
   checkForUpdates(false);
 }
 
+void MainWindow::reportSettingsVersionMismatchString()
+{
+  const QString mismatch_str = QString::fromStdString(m_host_interface->GetSettingsVersionMismatchString());
+  if (!mismatch_str.isEmpty())
+    reportError(mismatch_str);
+}
+
 void MainWindow::updateDebugMenuVisibility()
 {
   const bool visible = m_host_interface->GetBoolSettingValue("Main", "ShowDebugMenu", false);