efl_ui_slider_interval is totally different widget from efl_ui_slider.
Now, it extends efl_ui_layout_base and implements all efl_ui_slider's
functionalities by its own.
This class needs to be marked to @beta till reviewing whole class
definitions again.
@ref T7893
Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D9696