Fixed bug 5569 (Desktop should not show up in history list)

	I also prevented Control-B on the desktop from adding the
	desktop to the Bookmarks list.

	* src/file-manager/nautilus-desktop-icon-view-ui.xml:
	overrode command "Add Bookmark" to be insensitive.

	* src/nautilus-window.h: Declare function pointer for
	add_current_location_to_history_list.

	* src/nautilus-window-private.h: Declare private function
	nautilus_window_add_current_location_to_history_list.

	* src/nautilus-window.c:
	(nautilus_window_initialize_class),
	(real_add_current_location_to_history_list): Fill in
	add_current_location_to_history_list function pointer
	with simple function.
	(nautilus_window_add_current_location_to_history_list):
	wrapper for calling the new virtual function.
	(nautilus_forget_history):
	Call nautilus_window_add_current_location_to_history_list
	where it was doing the work directly, to get the indirection.

	* src/nautilus-window-manage-views.c: (update_for_new_location):
	Call nautilus_window_add_current_location_to_history_list
	where it was doing the work directly, to get the indirection.

	* src/nautilus-desktop-window.c:
	(nautilus_desktop_window_initialize_class),
	(real_add_current_location_to_history_list): Override
	add_current_location_to_history_list to do nothing.