Renamed QQuickItem::pos property to position

Abbreviated property names are less descriptive so we don't have
many of them.  Might as well be consistent.  QWindow::pos was already
renamed.

Change-Id: Ib52673e68e7dc902b2f8942dba6b899074b2538b
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>