Improve sorting in the bookmark list buffer

Improve sorting in the bookmark list buffer

  - Ensure that the bookmark bmenu buffer sorts when it should.
  - Sort case-insensitively and by locale-dependent collation order.
  - Rename "Bookmark" column to "Bookmark Name".
  - Coordinate that column's sort toggle and `bookmark-sort-flag'.
  - Document the new behavior.

* lisp/bookmark.el (bookmark-bmenu--name-predicate,
bookmark-bmenu--type-predicate, bookmark-bmenu--file-predicate): Use
`string-collate-lessp' with IGNORE-CASE argument, instead of plain
`string<'.
(bookmark-bmenu--revert): Sort based on `bookmark-sort-flag'.
(bookmark-bmenu-mode): Document the new behavior.   Rename the
"Bookmark" column to "Bookmark Name" for clarity & documentabilty.