ListView: add support for "pull back" header & footer

[ChangeLog][QtQuick][ListView] Introduced headerPositioning and
footerPositioning properties to control whether header and footer
are positioned inline, as overlays, or so that they slide away and
can be pulled back regardless of the content position.

Change-Id: Ifef1faf1ce6acf2b55cd1b6408e22ec2de841409
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>