FormWidget: Move aria attributes to this.focusNode.

Handles attributes specified declaratively at widget creation time,
plus set programatically (at any time) via setAttribute().

Also overrides getAttribute(), hasAttribute(), and removeAttribute()
to make it appear that the aria attributes are set on the widget root node.
The abstraction is leaky though because myWidget.attributes will reflect
the true attributes set on the root node.

Fixes #193.