Follow Qt style of implicitWidth/implicitHeight declaration

The question is, if this kind of (formatting only) change is welcome at all, should it be all the files at once, or they can be done in batches without waiting for a complete refactoring?

Mind you, that to the best of my knowledge some implicit sizing code in Kirigami needs a rewrite -- not just reformatting. I'll leave those for a separate careful review.

Blocked by: !384 (merged)