rename _WidgetBase --> Widget
rename _Contained --> Contained, _Container --> Container
rename _CssStateMixin --> CssState
rename _HasDropDown --> HasDropDown
rename _BidiMixin --> Bidi
rename _KeyNavMixin --> KeyNav
Rename _FormWidgetMixin --> FormWidget, _FormValueMixin --> FormValueWidget.
Also remove form/_FormWidget and form/_FormValueWidget.
rename _Invalidating --> Invalidating