Add a last-modified field when a bookmark is set

Add a last-modified field when a bookmark is set

* test/lisp/bookmark-tests.el (bookmark-tests-make-record)
(bookmark-tests-make-record-list, bookmark-tests-set): fix tests
to not consider last-modified in bookmark equality.
* lisp/bookmark.el (bookmark-make-record-default): add a
last-modified field.
(bookmark-sort-flag): add the 'last-modified choice.
(bookmark-get-last-modified): new function to get last-modified
bookmark field.
(bookmark-maybe-sort-alist): sort in last-modified first order.
(bookmark-completing-read): use `bookmark-maybe-sort-alist'.