Fix alphabetic sorting for headlines, tags

Fix alphabetic sorting for headlines, tags

* org.el (org-sort-entries): Use collated sorting.
(org-tags-sort-function): Use collated sorting.
(org-string-collate-greaterp): Add helper-function to use as defcustom
option, since there is no ‘string-collate-greaterp’ in Emacs.

* org-compat.el (org-string-collate-lessp): Add proxy to fall-back on
string-lessp when string-collate-lessp is missing (Emacs ≤ 24).

* test-org.el (test-org/string-collate-lessp): Add test.
(test-org/sort-entries): Add regression test for non-ASCII inputs.

‘org-sort-entries’ and ‘org-tags-sort-function’ advertise alphabetic
sorting, but actually sort based only on character code.  This
produces non-alphabetic orderings of strings in non-ASCII locales.

E. g., German Umlauts “Ä Ü Ö” are alphabetically sorted as if they
were “A U O”, whereas sorting based on character-code will place them
after “Z”, which is unexpected.