ibuffer: Display column titles in header line

ibuffer: Display column titles in header line

If the option `ibuffer-use-header-line' is set to `title',
display column titles in the header line.
* lisp/ibuffer.el (ibuffer--format-title)
(ibuffer--format-summary): New functions extracted from
`ibuffer-update-title-and-summary'.
(ibuffer-update-title-and-summary): Use them.
(ibuffer-update): Do not always override `header-line-format'.
(ibuffer-use-header-line): Update docstring and option `:type'.
* lisp/ibuf-macs.el (define-ibuffer-sorter): Add "@" to the
interactive specification for clicks on the header line.

* etc/NEWS: Announce the change.
(Bug#75497)

(cherry picked from commit 5294d450da47734bbe2ca259505674bcae5ff573)