Fix: ignored step value during dragging

[BartoszKlonowski]

This pull request fixes #390
It allows to successfully use the step property which is considered during dragging and during programmatically control of the Slider.

The root cause of this issue was that the controlled value property modified the dragging logic changing the way the value is passed to RNCSlider instance according to sliding start and progress.
Due to that the value was given the unclipped value (uncalculated) during dragging which was assigned to the thumb only, but at the finish of the dragging the value of the whole component received the calculated one.
This resulted in observed smooth thumb transition, while value having the values respecting the step property.

The implementation of the fix moves the calculation of the discrete value which calculates the value according to the given step (if no step is given, the transition remains smooth).
The internal discreteValue calculation is utilized in both Slider Manager and Slider instance itself.

The results of the fix are presented below:

390-repro-fix.mov