Yelp fixes for Mallard

[Shaun McCance <shaunm gnome org>]

I've just made some commits to Yelp to fix some Mallard
issues.  First, I made Yelp able to find Mallard documents
from ghelp URIs.  If the document foo is installed under
/usr/share/gnome/help/foo you can use ghelp:foo to get the
document.  (This respects XDG_DATA_DIRS, by the way.)  To
get the page bar, use ghelp:foo#bar.  There's currently
no way to scroll to a section in a page using a ghelp URI.

I've also fixed the image loading problem, sort of.  I can
guarantee that images will be correctly loaded if you load
the document from a ghelp URI.  If you pass a directory on
the command line, you MUST end it with a trailing slash to
get images.  It's kind of sucky, but it's just not worth
my time right now.  I have a new URI parser in the works.

Please continue banging on Mallard documents in Yelp and
let me know what problems arise.

--
Shaun