Also reduced the number of times that refresh_bookmarks_menu
	is called since it was showing up in some profiling.

	* libnautilus-extensions/nautilus-bookmark.h:
	* libnautilus-extensions/nautilus-bookmark.c:
	(nautilus_bookmark_initialize_class), (nautilus_bookmark_set_name),
	(bookmark_file_changed_callback): Split the "changed" signal into
	"appearance_changed" and "content_changed". "appearance_changed" is
	emitted only if the name or icon change. "content_changed" is emitted
	if the URI changes.

	* src/nautilus-bookmark-list.c: (insert_bookmark_internal):
	Connect to both appearance_changed and content_changed.

	* src/nautilus-window-menus.c: (bookmark_holder_new):
	Refresh menu only on appearance_changed, not content_changed.

	* data/static_bookmarks.xml: Added "i-web" as icon name for
	all the built-in bookmarks, since this is the icon they will
	end up with after being visited. (Now, visiting a built-in
	bookmark will not cause...