enhance $tw.wiki.sortByList with list-before and list-after

Enhanced $tw.wiki.sortByList() to allow for relative order of certain list elements according to their list fields "list-before" or "list-after". If either one of these fields is present for a list item (tiddler), then it controls relative placement of this item in the list: the value of one of these fields specifies the item to which this item is to be placed either before or after.