elm_slider: Step set/get API additions.

elm_slider: Step set/get API additions.

Issue: As step size was hard coded in the widget, when slider indicator is dragged

using a key event or during accessibility the value was not exactly incremented/decremented

instead same value was shown more than1 time/values were skipped.

Solution: Two APIs added which will set/get step size.

Reviewer: SeoZ
Reviewer Comment: This is a manual merge of D293 due to arc issue on Shilpa.
I modified some codes from her original code. She'll use arc next time.
 fixed documentation
 fixed typos.
 fixed indentation
 used macro
 added some guide codes.
 removed unnecessary empty line.
 updated NEWS and ChangeLog.