Changes

Summary

1. ZF belongs to chapter FOL, following lib/html/library_index_content.template (i.e. "Documentation" area on website);
2. include groups from 'chapter_definition' in session info, based on the state of chapter_defs after processing all ROOT files (thus the declaration order does not matter);
3. proper chapter_definition to prevent odd definitions by users;
4. tuned;
5. tuned signature;
6. proper order;
7. tuned whitespace for presentation;
8. clarified signature of Sessions.Session: chapter_defs: vs. known_chapters (in declaration order) vs. chapters (covered by visible sessions); clarified browser_info root index: preserve declaration order as much as possible;
9. clarified syntax: more uniform;
10. more robust, more strict;
11. tuned signature;
12. tuned;