efl_ui_slider: refactor for better data flow and code readability

1. efl_ui_range_value_set only sets value
2. remove internal eo functions (such as xxx_knob, val_fetch, and val_set)
3. make access_value_and_text works correctly

@ref T7893

Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D9720