Give drawers their own invisibility spec

Give drawers their own invisibility spec

* lisp/org.el (org-mode): Add special invisibility spec for drawers.
(org-show-set-visibility): Handle new invisibility spec.
(org-clean-visibility-after-subtree-move): Fix argument for
`org-cycle-hide-drawers'.  `overview' is a no-op.
(org-cycle-hook):
(org-cycle-internal-local):
(org-flag-drawer):
(org-move-subtree-down):
(org-show-entry):
* lisp/org-agenda.el (org-agenda-clock-in):
* lisp/org-clock.el (org-clock-goto):
* lisp/org-feed.el (org-feed-update):
* lisp/org-inlinetask.el (org-inlinetask-toggle-visibility): Do not hide
  drawers when expanding a headline.

Drawers and headlines visibility are now unrelated.  As a consequence,
there is no need to hide drawers every time a headline is expanded.