In _introspect(), make sure "this" refers to the prototype.

Also changed _WidgetBase._introspect() to set properties directly in the prototype,
rather than on the constructor.
Make genSetter() map to DOMNode properties, not attributes.
Mapping to properties will work with DOMNodes that are widgets, and also with input.value.
It won't work for things like aria-disabled though, that must be set as attributes.
Support widgets in templates.

Part of this change is to make templated set the DOMNode property or the DOMNode
attribute, depending on which field you are trying to set.
Often setting either one is OK, but you need to set properties to
affect input.value and widgets, and you need to set attributes for
things like role, that have no associated property.