Remove underscore from names of protected properties:

* CustomElement: _created, _attached, _parseAttr --> parseAttribute, _parseFunctionAttr --> parseFunctionAttribute
* HasDropDown: _buttonNode, _arrowWrapperNode, _popupStateNode, _aroundNode
* KeyNav: _onLeftArrow, _onRightArrow, _onUpArrow, _onDownArrow, _getNext
* Widget: _started

Store still needs to be addressed.

Refs #280.
Use eventNameHandler or _eventNameHandler for event handlers.
Fixes #281.

Note that _onFocus/_onBlur/onFocus/onBlur is tracked separately as #214.
remove vestigial code from when scroll bar was on popup wrapper, refs #266.