elm_button should become efl_ui_button and have its interface properly cleaned.
I think that this work should be done sequentially after finishing T5363.
We need to think about the hierarchy between basic widget class and this button class.
(Such as new Layout Class, ...)
Button is nothing but a Layout(or View) Class with Clickable feature.
Of course, it needs to support some states (pressed, unpressed, ..) and internal text object (+ image object ? I don't know).
So, if we will make a good parents class for this New Button Class, this task only needs small work to do.