prefs.js: disable nautilus desktop icons

If the system has an old version of Nautilus file manager, one with support for painting the desktop icons, it is paramount to disble them in the settings, because not doing so results in interference between the icons drawn by Nautilus and the ones painted by desktop-icons extension.

This patch sets to FALSE, during startup, the settings key /org/gnome/desktop/background/show-desktop-icons, in order to ensure this.

Fixes #70 (closed)