This method adds an item to the end of the list.
You can add a Bookmark directly or you can use one of the
overloaded operators to add a bookmark with a specified title.
When you add a string this is encapsulated within a new Bookmark
which is then inserted.
Bookmarks can exist in only one place at a time. If the Bookmark
supplied is already contained by another object then a Clone of the
Bookmark is added.