BUG: Update qMRMLSliderWidget single step when range changes

The single step can depend on the range of the slider, update it when
needed.

Issues #3657, #3700
$ git shortlog 3aaa591..daaf705 --no-merges
Julien Finet (1):
      Virtualize ctkRangeWidget and ctkSliderWidget public API

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23224 3bd1e089-480b-0410-8dfb-8563597acbee