efl_ui_slider: refactor for better data flow and code readability
- efl_ui_range_value_set only sets value
- remove internal eo functions (such as xxx_knob, val_fetch, and val_set)
- make access_value_and_text works correctly
@ref T7893
Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D9720