ssite dump_meta: add the extra information tracked by special pages

[Enrico Zini]

dump_meta now writes data in human readable and parsable yaml/json/toml.

However, pages don't yet contribute to the metadata the extra information they possess beyond their metadata, like name for taxonomy category pages, or the list of syndicated pages for syndication page.

There's a need to do a review of all Page subclasses, and tweak their to_dict method to make sure its result has all that templates can use from the page.

Doing this could very likely result in a change of the structure of the data output by dump_meta, to have it match what a template can see. For example, page metadata should end up in a meta dict instead of being at top level.