Adding support for xsane

[Richard Hughes <hughsient gmail com>]

I've coded some initial support for xsane-based application in a
private branch. It basically writes the right thing in the per-user
xsane directories for the system and device settings. I'm not sure
whether to merge it to master or not, as it's pretty hacky code.
Ideally we would talk to easy-scan / gnome-scan in the future using a
nice DBus interface, but I think that's a few years away.

It does not depend on sane (that's a separate branch...) and if you
don't use xsane then the file just gets written and nothing uses it.

So, what do you guys think?

Richard.