Allow more types, fix bug with deep section nesting, fix bug with note lists and validate navigation document heading references

 * added validation that makes sure that all headlines in the book are referenced from the navigation document
 * spaces in headlines are now normalized in DTBook output (prevents leading and trailing space validation errors)
 * fixed conversion of note lists (again!), fixes #187 
 * fixed problem with using nesting depths of 6 or more in the sectioning hierarchy, fixes #254 
 * allow more epub:types in top-level sections, fixes #230. `titlepage` is still only allowed in frontmatter and `part` is still only allowed in bodymatter. Other than that, these types can be used in either frontmatter, bodymatter or backmatter: `chapter`, `abstract`, `foreword`, `preface`, `prologue`, `introduction`, `preamble`, `conclusion`, `epilogue`, `afterword`, `epigraph`, `toc`, `toc-brief`, `landmarks`, `loa`, `loi`, `lot`, `lov`, `appendix`, `colophon`, `credits`, `keywords`, `index`, `glossary`, `bibliography`, `titlepage`, `halftitlepage`, `copyright-page`, `seriespage`, `acknowledgments`, `imprint`, `imprimatur`, `contributors`, `other-credits`, `errata`, `dedication`, `revision-history`, `case-study`, `answers`, `assessments`, `qna`, `practices`, `footnotes`, `rearnotes`