Remove _AttachMixin, _TemplatedMixin, and _WidgetsInTemplateMixin.
They are essentially replaced by the handlebars plugin.
get _HasDropDown working again after ES5 accessors check in
remove deprecated code etc. from helpers.js
Stateful: fix comment
Use register() for declaring widgets and declare() for base classes.
Updated register() to search inherited properties too for shorthand notations
like _setFooAttr: "focusNode".