Headers and footers are given a default z value. This value
should be set between Component beginCreate() and completeCreate()
to avoid overwriting any binding in the header or footer item
that has already set the 'z' value.

Change-Id: I8e3ee97232d99500a32f2d77d2b663016eb404f7
Reviewed-by: Martin Jones <martin.jones@nokia.com>