Make invisible targets anchors in the headlines or items.

Make invisible targets anchors in the headlines or items.

Org allows to define invisible anchors in a document in comment lines
like

If such a line is directly before or after a headline, previously this
anchor was removed, and all references to it were replaced with
references to the anchor that the headline had anyway.

Matthew Lundin pointed out that this makes it impossible to have
permanent links to headlines that can also be used from outside the
current page.

This patch changes his situation by adding the additional targets as
empty anchors to the section heading.  If works by creating, during
preprocessing, an alist with targets that do have aliases.  During
publishing, these aliases are looked up and converted into anchors.
In LaTeX export, these additional targets become additional labels.