[BUGFIX] Slider Wizard value not set when used in a flexform

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: I59d967bc2a782f7f3f392c1fe035dc0c60ef6e75
Fixes: #34012
Releases: 4.5,4.6,4.7,6.0
Reviewed-on: http://review.typo3.org/11038
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch