org-list: implement new accessors to list structures

org-list: implement new accessors to list structures

* lisp/org-list.el (org-list-get-all-items): new function
(org-list-get-all-children): new function
(org-list-get-nth): new function
(org-list-set-nth): new function
(org-list-get-ind): new function
(org-list-set-ind): new function
(org-list-get-bullet): new function
(org-list-set-bullet): new function
(org-list-get-checkbox): new function
(org-list-set-checkbox): new function
(org-list-struct-fix-bul): use new accessors
(org-list-repair): use new accessors
(org-list-indent-item-generic): make use of accessors
(org-list-get-parent): renamed from org-list-struct-get-parent
(org-list-get-child): renamed from org-list-struct-get-child
(org-list-struct-fix-ind): make use of accessors
(org-list-get-next-item): new function
(org-list-get-subtree): new function