Update the implicitWidth of the item before continuing with the layout
rather than after it's completed this gives any bindings dependent on
the implicitWidth the opportunity to update the width before the layout
continues.

Task-number: QTBUG-22680
Task-number: QTBUG-24251
Change-Id: I61cd96ad9891b22d8b83937ad2c06719f88976b6
Reviewed-by: Martin Jones <martin.jones@nokia.com>