Fix ctkSliderWidget and ctkRangeRidget valueChanged signal

The issue was only when slider tracking is false.

Thanks to magro11 for the bug report & fix
Closes #444
Merge branch '444-fix-slider-valueChanged-with-no-tracking'

* 444-fix-slider-valueChanged-with-no-tracking:
  Fix ctkSliderWidget and ctkRangeRidget valueChanged signal