🍒KF5: Delegates: Set padding values in a smarter way

cherry picked from MR !245 (merged)

CC @ndavis @carlschwan

To be precise, it's a behavior fix, not a refactor. Can be seen as an added support for newer QtQuick features. So I'm 50/50, but sounds like this is not welcome in kf5, so I won't pursue it.

I'll let community decide whether this should go into KF5 branch.