change thumbnailer spec to fit better with xdg-basedir spec

[Dr. Michael J. Chudobiak]

Martin Meyer wrote:
> (cached) data". I feel that the thumbnail cache would fit in there. Is
> there a chance of getting the spec changed to point to
> $XDG_CACHE_HOME/thumbnails instead perhaps? Is it even open for
> discussion?

It sounds like we have a positive consensus...

I am working on creating a new docbook version of the thumbnail spec. 
Once it is ready, I'll propose a patch to reflect the movement of the 
thumbnail cache.

- Mike