change thumbnailer spec to fit better with xdg-basedir spec

[Dr. Michael J. Chudobiak]

> Or rather in GLib, g_get_user_thumbnails_dir(), since it's the least
> common denominator in that group. Then again this function probably
> should have existed for quite some time, since its present-day
> counterpart is synthesized in all of those places mentioned above. So
> then as a first step for us, we could move the existing code to e.g.
> "g_get_user_thumbnails_dir()" in GNOME to make this transition easier,
> if that's what we all agree on. And even if it doesn't end up
> happening now, at least we're futureproof.

I have proposed a g_get_user_thumbnail_dir patch here:

http://bugzilla.gnome.org/show_bug.cgi?id=528749

Please review!

- Mike