Screenshots

[John Darrington]

I've pushed a change which adds some screenshots to the user manual,
along with a framework to autogenerate them.  The sad reality is
however, that while it works automatically, I think it is not ever
going to work in a portable fashion.  So I've checked in the generated
images.

To regenerate them on a debian system the procedure is:

apt-get install xdotool xvfb imagemagick x11-apps
make doc-make
make -f doc-make

Feedback is welcome.