Update property API

Update property API

* lisp/org.el (org-entry-put): Refactor code, taking into account
  changes to property drawer syntax.  Fix errors when handling special
  values, which cannot be symbols.  Remove CLOCKSUM handling.
(org-entry-get): Refactor code.
(org-entry-delete): Small refactoring, do not rely on
`org-remove-empty-drawer-at' since parser is not necessary here.
(org-buffer-property-keys): Fix infloop.  Ignore final "+" in extended
properties.  Refactor code.
(org-property-values): Include extended values.
(org-entry-get-with-inheritance, org-insert-property-drawer): Small
refactoring.
(org-insert-drawer): Fix docstring and comments.
(org--update-property-plist): Renamed from org-update-property-plist.
Use side effects.  Improve speed.

CLOCKSUM special property in `org-entry-put' is buggy (symbols instead
of strings) and ignores provided value.  Since the expected behaviour
is neither clear nor documented, the property is not handled anymore
by the function.