Add API to make header and footer paddings optional

Part of teams/vdg/issues#47

This facilitates KCMs that want to touch the edges of the page with header or footer content. For example, an InlineMessage with the position property set to "header."

SimpleKCM only gets the header property since it doesn't have special handling for a footer. Also for SimpleKCM the padding defaults to off rather than on, replicating the status quo (which appears to be the result of a change we forgot to make for KF6, but that's a topic for another time).