Expose QQuickItem::window as a property

Everything is already in place, except that for some reason there is
no property. Add it because this is the most efficient and robust
way for Quick Controls to figure out which window a given item belongs to.

Change-Id: I41b513e99d924f3e56346f8480df50a77c16e8bf
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>