Factorize tags parsing
* lisp/org.el (org-tag-string-to-alist):
(org-tag-alist-to-string):
(org-tag-alist-to-groups): New functions.
(org-set-regexps-and-options): Use new functions.
(org--setup-process-tags): Remove function.
(org--setup-collect-keywords): Return tag groups as a string instead of
a list so as to be compatible with new functions.
* lisp/org-mobile.el (org-mobile-create-index-file): Use new functions.
* lisp/org-pcomplete.el (pcomplete/org-mode/file-option/tags): Use new
functions.
(pcomplete/org-mode/tag): Small refactoring.
* testing/lisp/test-org.el (test-org/tag-string-to-alist):
(test-org/tag-alist-to-string):
(test-org/tag-alist-to-groups): New tests.