MB-9183: added ability to customize z-index of spinner

Change-Id: I15b47087735fe11fb24e9f470c763577b85e651c
Reviewed-on: http://review.couchbase.org/29371
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>