Quickbook: Deal with checked paths that aren't present. Refs #6691.

I think that the dependencies should just be files that were loaded, so
I changed it to do that, but added an extra (hidden for now) option to
list all paths that were checked, staring with a `+` for files that are
present, and `-` for files that are absent.

Also fixed a bug with the path resolver.
Quickbook: Remove intrusive_base. Refs #6794.
Quickbook: Set version for release.
Quickbook: Fix documentation.

Was using a feature I haven't implemented yet.
Quickbook: Backdated changelog for 1.5.8.

Oops, I completely forgot to write this up at the time.
Quickbook: Describe new 1.6 tags.
Quickbook: Fix typo.
Bump version number.
Convert to use boost::string_ref
Source map tests.
Store indentation change in source map.

Used to infer it when required, which could go wrong.
Fix `to_original_pos` for indented blocks.
Clean up poorly overloaded 'add' method.
Extract duplicated find_section code.
Fix mapped position when breaking up indented code.
Remove whitespace only leading lines from indented blocks.
Move dependency tracking into a separate class.
Split path normalization from 'add_dependency'.
Changelog note for quickbook 1.5.9
Better argument parsing.