The attached patch against r59286 tries to tease apart the uses of
filename by adding "webify_filepath" and "unwebify_filepath" functions
to sphinx.utils which are then used throughout the app to convert from
filesystem-separated to web-separated paths and back. A WEB_SEP constant
has been defined principally to make its purpose clear in case a
constant "/" was thought to have been a hangover from the code's *nix
origins.

The result has been tested by building manually, ie by invoking python
tools\sphinx-build.py with appropriate params under Win32 native only.
The html, web and htmlhelp versions all style up and function correctly.
It would obviously benefit from testing under Cygwin and *nix. I'm
particularly keen to see it it works on a filesystem which doesn't allow
"/" as a separator.