Second draft of python binding of Canvas:setPreferredCenter()

This is in response to @dkazakov's comments on my first merge request in !2298 (comment 1139602) . (I thought that A force push to the same branch would update the previous merge request, but apparently intent.kde.org doesn't work like that.)

This is my second attempt, I have implemented all of @dkazakov comments to the best of my understanding, and I'm looking forward to a second review.