KWindowStateSaver: Increase the rate limit on the slow part of config saving

KConfig::writeEntry is a super-cheap operation. It's changing something in a dictionary. It's not that important to limit it.

KConfig::sync is an extremely expensive operation. It blocks until things are physically on the storage media.

The current code needed the timer to be a very short value because you don't know when the app is going to quit and we need the config object up-to-date. On the other hand syncing to disk every 15 frames is way too fast, it stood out whilst tracing frame drops.

We don't need to sync to disk explicitly at all, it will be synced when the app quits. We do something anyway, just in case the app happens to crash. This can be at a far more leisurely place as it's not very important.