[GUI] Slider in Postvis should fast-forward and rewind immediately

Problem to solve

After using the new FootStepProcessor for postvisualization, the slider in the Postvis GUI should fast-forward and rewind the simulation immediately while the slider is dragged (like before merge 696a81f4) and not just after releasing the slider. See following animation of the old GUI behavior:

Further details

This allows quicker visual debugging.

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