at Bud's request, implemented bug 4127, add a preference to
	show or hide Nautilus' desktop.

	* libnautilus-extensions/nautilus-global-preferences.h:
	define NAUTILUS_PREFERENCES_SHOW_DESKTOP
	* libnautilus-extensions/nautilus-global-preferences.c:
	(global_preferences_create_dialog), (global_preferences_register):
	register the desktop preference and show the checkbox in
	the appearance section of preferences
	* src/nautilus-application.c: (nautilus_application_initialize),
	(nautilus_application_destroy), (check_for_and_run_as_super_user),
	(desktop_changed_callback):
	watch the desktop preference and show or hide the desktop window
	when it changes.
	* src/nautilus-main.c: (main):
	show the desktop at launch if the preference is set and it's not
	overriden by command-line options.