Allow messages to be in center

We’ll also need an update hook to update config in existing installations.

Add hook update for existing config to set to current values.

Please commit any new changes to the merge request instead of posting patches. I fixed some issues that were introduced and I removed the last references to the positionLeft option. @lamp5 are you okay with that? There's an update hook that migrates existing config to the position option.

Thanks everybody!

Automatically closed - issue fixed for 2 weeks with no activity.