Allow a custom headline prefix in orgstruct-mode.

Allow a custom headline prefix in orgstruct-mode.

* lisp/org.el (org-cycle-global-status)
(org-cycle-subtree-status): Set state property.
(org-heading-components): Use org-heading-regexp in
orgstruct-mode.
(orgstruct-heading-prefix-regexp, orgstruct-setup-hook): New
options.
(orgstruct-initialized): New variable.
(orgstruct-mode): Simplify implementation.
(orgstruct-setup): Simplify implementation.  Translate keys to
their most general equivalent.
(orgstruct-make-binding): Generate index on the fly.  Discard
alternative keys.  Bind variables according to
orgstruct-heading-prefix-regexp.
(org-get-local-variables): Honour state property.
(org-run-like-in-org-mode): Do not override variables with
non-default values.
(org-forward-heading-same-level): Do not skip to headlines on
another level.  Handle negative prefix argument correctly.
(org-backward-heading-same-level): Use
org-forward-heading-same-level.