gnome-desktop: update to latest version of upstream thumbnail code

The thumbnail code in gnome-desktop had recent bug fixes that needed to be carried across to the Nautilus implementation. Issues for the three related upstream fixes can be found at: gnome-desktop#4 (closed) | gnome-desktop#89 (closed) | gnome-desktop#87 (closed)

This patch includes the latest thumbnail code from the gnome-desktop repository, incorporating the fixes.

Fixes #836 (closed)