Simplify `org-show-context' configuration

Simplify `org-show-context' configuration

* lisp/org.el (org-show-context-detail): New variable.
(org-context-choice, org-show-following-heading, org-show-siblings,
org-show-entry-below, org-show-hierarchy-above): Remove variables.
(org-show-set-visibility): New function.
(org-convert-to-oddeven-levels, org-get-location, org-show-context,
org-reveal): Use new function.
(org-link-search): Update docstring.

* lisp/org-agenda.el (org-agenda-cycle-show): Use new function.
(org-agenda-show): Replace missing variable.

* testing/lisp/test-org.el (test-org/show-set-visibility): New test.

* doc/org.texi (Sparse trees):
* doc/orgguide.texi (Sparse trees): Document changes.

* etc/ORG-NEWS: Signal changes.

Configuration of `org-show-context' is done with a single variable
offering six different views, instead of four variables for a total
of 16 configurations.