A widget that implements this interface has the ability to override
the document directionality for its root element.
Widgets that implement this interface should be leaf widgets. More
specifically, they should not implement the
Possible return values for getDirection() and parameter values for
setDirection(Direction).Widgets that implement this interface can
either have a direction that is right-to-left (RTL), left-to-right (LTR), or default
(which means that their directionality is inherited from their parent widget).