Quickbook: Keep files in memory, stop tracking position in iterators.

Files are now permanently loaded. Quickbook substrings are stored as
references into the file. Now positions are stored as iterators into
the original file, the line and column is calculated when messages are
output.

This doesn't have much effect on efficiency but it simplifies a few
things.

[SVN r75253]

src/template_stack.cpp

@@ -18,11 +18,9 @@ namespace quickbook
 {
     template_body::template_body(
             value const& content,
-            fs::path const& filename,
             content_type type
         )
         : content(content)
-        , filename(filename)
         , type(type)
     {
         assert(content.get_tag() == template_tags::block ||