Slider: extract distance calculation logic into dedicated methods

Fixes #10737 

~ Extracted the code responsible for translation of values into slider handle's distances into _calculateDistance() (was in _calculateDistance())
~ Extracted the code responsible for translation of slider handle's distance into numeric values into _calculateValue() (was in _normValueFromMouse())
~ Added two new callback options and modified the constructor to set the new methods as default values (if not already set)
~ cleaned up all variables that became unused due to the re-factoring
~ updated the common test case to reflect the change in options