Add attached Window::contentItem property

This is essential to the Qt Quick Controls for implementing floating
text selection handle popups.

Change-Id: Ibae65110a5db6d65dacd197fef3ad567d11342dc
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>