[patch] thumbnail cache purge function

["Dr. Michael J. Chudobiak" <mjc avtechpulse com>]

It would be nice to have a function which would delete files in 
~/.thumbnails based on two gconf parameters, something like:

/desktop/gnome/thumbnail_cache/max_size (default = 256 MB?)
/desktop/gnome/thumbnail_cache/max_age (default = 90 days?)

Then, gThumb could call this function occasionally (with a UI to set the 
two keys), or better yet, gnome-session or gdm could call it on logout 
(or is there a better way?).

There is currently no cache cleaning in gnome, and large caches slow 
down login noticeably (bug 430123).

Possible patch here:
http://bugzilla.gnome.org/attachment.cgi?id=107622&action=view
http://bugzilla.gnome.org/show_bug.cgi?id=523159

- Mike