fix sections with h6
validate that all headlines in the book are referenced from the navdoc
allow more epub:types in top-level sections
fixed epub examples according to nordic_nav_references_1
check that text in nav toc corresponds to content headlines

(updated test files as well)
fixed conversion of note lists (again)
normalize space in headlines in DTBook output
Merge pull request #256 from nlbdev/h6-navrefs-types

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