fixed bug 5338, switching to and from "tighter layout" messes up
	icon labels, by adding a call to invalidate an item's cached label
	size, and making the icon canvas invalidate the items when appropriate.
	This also fixes a similar problem when the anti-aliased mode changes.

	* libnautilus-extensions/nautilus-icon-canvas-item.c:
	(nautilus_icon_canvas_item_initialize),
	(nautilus_icon_canvas_item_invalidate_label_size),
	(nautilus_icon_canvas_item_set_arg):
	renamed internal invalidation routine to make it externally accessible

	* libnautilus-extensions/nautilus-icon-canvas-item.h:
	added nautilus_icon_canvas_item_invalidate_label_size

	* libnautilus-extensions/nautilus-icon-container.c:
	(invalidate_label_sizes),
	(nautilus_icon_container_set_anti_aliased_mode),
	(nautilus_icon_container_set_tighter_layout):
	added an invalidate_label_sizes routine and called it when
	tighter layout or the anti-aliased mode changes.