id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
11771	Introduce styleableElement in widget API	Olek Nowodziński		"At the moment (#11297) to change the way widgets are styled, developer must override `widget#add/has/getClasses`, which is not quite straightforward. 

The idea is to introduce `widget#styleableElement` property which, referring to the right element, can be re-used in `widget#add/has/getClass` and reduce the complexity of customisation.

Problems:
* How to set it? Via `widgetDef` (selector, method...)? Via `widget.setStyleableElement`? By overriding `widget.getStyleableElement`?
* How to alter it? `delete widget.styleableElement`? `widget.setStyleableElement`? By overriding `widget.getStyleableElement`?
* How to optimise it? Cache? How to override cached element?"	Bug	confirmed	Normal		UI : Widgets	4.4.0