Patch for 78542 - change thumbnails to use a thread

[Damon Chaplin <damon ximian com>]

I've added a monster patch to:

http://bugzilla.gnome.org/show_bug.cgi?id=78542


Nautilus currently does a fork() to create each thumbnail, which I don't
think is safe when using gnome-vfs, and it is also inefficient.

The patch changes it so it uses a thread to create all the thumbnails in
a directory. (I assume using pthread functions is OK in nautilus. They
are used elsewhere.) Though it still forks/execs 'convert' if it has to.
The patch also removes code related to public metadata, which we don't
use any more.


I don't know if this fixes the particular bug. But convert works for me
now, and it didn't before.

Damon