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.

[SVN r77973]
Quickbook: Remove intrusive_base. Refs #6794.

[SVN r77998]
Quickbook: Set version for release.

[SVN r78727]
Quickbook: Fix documentation.

Was using a feature I haven't implemented yet.

[SVN r78728]
Quickbook: Backdated changelog for 1.5.8.

Oops, I completely forgot to write this up at the time.

[SVN r82573]
Quickbook: Describe new 1.6 tags.

[SVN r82589]
Quickbook: Fix typo.

[SVN r83018]
Bump version number.

[SVN r83115]
Convert to use boost::string_ref

[SVN r83116]
Source map tests.

[SVN r83117]
Store indentation change in source map.

Used to infer it when required, which could go wrong.

[SVN r83118]
Fix `to_original_pos` for indented blocks.

[SVN r83119]
Clean up poorly overloaded 'add' method.

[SVN r83120]
Extract duplicated find_section code.

[SVN r83121]
Fix mapped position when breaking up indented code.

[SVN r83122]
Remove whitespace only leading lines from indented blocks.

[SVN r83123]
Move dependency tracking into a separate class.

[SVN r83124]
Split path normalization from 'add_dependency'.

[SVN r83125]
Changelog note for quickbook 1.5.9

[SVN r83130]
Better argument parsing.

[SVN r83134]