fix error deleting selected RadioMenuItem, fixes #17519
(cherry picked from commit 57966f299a6c0b58933bad5566525cb268eccbaf)
Remove tag and declaredClass from proto.
The widget class should be obvious from this.nodeName.
Omit properties like "style" from this._propCaseMap.

Rearchitected Stateful to have a _getProps() method that returns all the properties
that should be watchable, or in other words, all the "official" properties.

_WidgetBase._getProps() has the code to only look at properties defined in the widget
definitions, thus ignoring properties in HTML*Element base classes.