Properly save custom header data

Ensure adding custom data upgrades to KDBX4
Implement review feedback

src/gui/WelcomeWidget.cpp

@@ -73,4 +73,4 @@ void WelcomeWidget::refreshLastDatabases()
         itm->setText(database);
         m_ui->recentListWidget->addItem(itm);
     }
-}
+}