Additions to implement colors from Quickbook 1.6.

But align feature (done in boostbook.css for html) is still not implemented.

[SVN r86229]
Add assignment return to doxygen2boostbook. Refs #9241.

[SVN r86252]
Fix virtual methods in doxygen2boostbook.

[SVN r86270]
Add newlines to doxygen2boostbook output.

Hopefully this won't break anything, and will make it much easier to read the
intermediate files.

[SVN r86271]
Remove unused date handling code from html-help.

[SVN r86272]
Try to make last-revsion a little more robust.

Don't display anything for empty '$Date$' variables, and just display the value
for normal, non version control values.

[SVN r86273]
Factor out method and function attributes.

Not all attributes apply to all methods and functions, but hopefully they should
never appear when inappropriate.

[SVN r86274]
Avoid confusing arguments with quialifiers from argsstring.

[SVN r86275]
Less verbose handling of qualifiers.

[SVN r86276]
Add noexcept and constexpr to doxygen to boostbook coversion.

Should really add proper support to boostbook for `=delete`, `=default` and
noexcept, but I'm not feeling that keen right now. I'm not sure I did the right
thing with `BOOST_CONSTEXPR_OR_CONST`.

[SVN r86277]
Fix support for const, volatile, and pure virtual.

[SVN r86278]
Use specifiers attribute for virtual and explicit.

[SVN r86279]
Parse brackets in argsstring.

This may seem like overkill, but it's going to be needed for handling
parameters to noexcept. Doesn't handle escapes because that my sanity is
already stretched far enough.

[SVN r86284]
Support noexcept with a conditional.

Turns out Doxygen doesn't like BOOST_NOEXCEPT_IF. Text parsing in XSL is an
almighty pain, it might be better to just combine whatever follows the
arguments in argsstring with the values from the attributes.

[SVN r86285]