xhtml: move parse_hidden_node logic to parse_node

The hidden nodes should not generate any event-based content
(begin/end context, text, etc.) but may generate structural events
(anchor, etc.) and RDF metadata (RDFa, ePub3 navigation, etc.).

The new structure is to store a visible flag on the context_data for
each block scope and to parse the hidden nodes within the main
parsing logic. This allows supporting the structural events and
metadata, while making it easier to support the @hidden attribute.