When the Slider Wizard is used in a flexform the value is given
as XML, which is not handled correctly. When the Slider is used in
a flexform, retrieve the value with t3lib_flexformtools.

Change-Id: Ifeb9f29fe356d062f027f7fba32f912d66743e24
Fixes: #34012
Releases: 4.5,4.6,4.7,6.0
Reviewed-on: http://review.typo3.org/9932
Reviewed-by: Claus Due
Reviewed-by: Arjen Hoekema
Tested-by: Arjen Hoekema
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch