emacs: rename `notmuch-show-toggle-headers' to `notmuch-show-toggle-visibility-headers'

emacs: rename `notmuch-show-toggle-headers' to `notmuch-show-toggle-visibility-headers'

* emacs/notmuch-show.el

  (notmuch-show-toggle-headers):
    Rename to `notmuch-show-toggle-visibility-headers'.

  (notmuch-show-mode-map):
    Update "h" binding wrt renamed `notmuch-show-toggle-headers'.

  (notmuch-message-headers):
    Update docstring wrt renamed `notmuch-show-toggle-headers'.

  (notmuch-message-headers-visible):
    Update docstring wrt renamed `notmuch-show-toggle-headers'.
    Also fixed a small typo.

* test/emacs:

  Update subtest wrt renamed `notmuch-show-toggle-headers':
  - "notmuch-show: hide message headers (w/ notmuch-show-toggle-headers)"