Remove the header line after disabling 'which-function-mode'

Previously, the header line would stay around even when after
disabling 'which-function-mode', although it may be empty.  Now
the 'which-function-mode' element is properly removed from
'header-line-format', so the header line will disappear if
there's nothing else in 'header-line-format'.

Also, previously, when we ran (which-function-mode), we would
enable
'which-function-mode' for all buffers even if they didn't support
imenu.  We didn't run the normal logic in 'which-func-ff-hook' to
disable 'which-func-mode' if imenu wasn't present.  Now we do run
that logic, by just calling 'which-func-ff-hook'.  This is
especially important when the header line is enabled, because
otherwise there's a very noticeable header line added to every
buffer, including e.g. *Help* and *Buffer List*.

Also, we now check that 'header-line-format' is a list before trying
to add to it; this makes us work properly when enabling and
disabling 'which-function-mode' for modes which set
'header-line-format' to a string or symbol, such as eww.

* lisp/progmodes/which-func.el (which-func-try-to-enable): Re-add
'which-func-format' to the header line.
(which-func--header-line-remove): New function.
(which-func--disable): Call 'which-func--header-line-remove'.
(which-function-mode): Call 'which-func-ff-hook' and
'which-func--header-line-remove'. (bug#66283)
* test/lisp/progmodes/which-func-tests.el: New test.

test/lisp/progmodes/which-func-tests.el

@@ -0,0 +1,58 @@
+;;; which-func-tests.el --- tests for which-func     -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2023 Free Software Foundation, Inc.
+
+;; Author: Spencer Baugh <sbaugh@catern.com>
+
+;; This file is part of GNU Emacs.
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+(require 'ert)
+(require 'which-func)
+
+(ert-deftest which-func-tests-toggle ()
+  (let ((which-func-display 'mode-and-header) buf-code buf-not)
+    (setq buf-code (find-file-noselect "which-func-tests.el"))
+    (setq buf-not (get-buffer-create "fundamental"))
+    (with-current-buffer buf-code
+      (should-not which-func-mode) (should-not header-line-format))
+    (with-current-buffer buf-not
+      (should-not which-func-mode) (should-not header-line-format))
+    (which-function-mode 1)
+    (with-current-buffer buf-code
+      (should which-func-mode) (should header-line-format))
+    (with-current-buffer buf-not
+      (should-not which-func-mode) (should-not header-line-format))
+    (which-function-mode -1)
+    ;; which-func-mode stays set even when which-function-mode is off.
+    (with-current-buffer buf-code
+      (should which-func-mode) (should-not header-line-format))
+    (with-current-buffer buf-not
+      (should-not which-func-mode) (should-not header-line-format))
+    (kill-buffer buf-code)
+    (kill-buffer buf-not)
+    (which-function-mode 1)
+    (setq buf-code (find-file-noselect "which-func-tests.el"))
+    (setq buf-not (get-buffer-create "fundamental"))
+    (with-current-buffer buf-code
+      (should which-func-mode) (should header-line-format))
+    (with-current-buffer buf-not
+      (should-not which-func-mode) (should-not header-line-format))))
+
+(provide 'which-func-tests)
+;;; which-func-tests.el ends here