Match `org-complex-heading-regexp' with a nil `case-fold-search'

Match `org-complex-heading-regexp' with a nil `case-fold-search'

* lisp/org-agenda.el (org-agenda-goto):
* lisp/org-clock.el (org-clock-in):
(org-clock-out):
(org-clock-put-overlay):
(org-clock-load):
* lisp/org-element.el (org-element-context):
* lisp/org-footnote.el (org-footnote--allow-reference-p):
* lisp/org-mobile.el:
* lisp/ox.el (org-export--get-subtree-options):
* lisp/org.el (org-insert-heading):
(org-edit-headline):
(org-open-at-point):
(org-refile-get-targets):
(org--get-outline-path-1):
(org-toggle-comment):
(org-set-tags-to):
(org-set-tags):
(org-entry-properties):
(org-delete-indentation):
(org-beginning-of-line):
(org-end-of-line):
(org-mode-flyspell-verify): Bind `case-fold-search' to nil when matching
`org-complex-heading-regexp'.

(org-complex-heading-regexp): Add a note about the necessity to have
`case-fold-search' bound to nil.