Fix preferences references

We were using the wrong spellcheck preferences, they have moved to the
spellcheck component. It mostly worked because their internal names had
stayed the same.

Change-Id: I485b11d88ee9c6e13d9c2b769c271d27bee4ba30
Reviewed-by: Michal Klocek <michal.klocek@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>