update CssState tests following css renaming
register(): don't require _introspect() method.

register() should take any class.
The class can optionally have createdCallback() and enteredViewCallback() methods,
but it shouldn't need an _introspect() method.

Refs #81.
parse() should just no-op when no widgets are registered
remove unused file and reference to image that no longer exists
fix a11y test failure (on FF)