[O] ox-texinfo: use an Org section as Info Top node

[Jonas Bernoulli]

Hello,

Congratulation on converting the Org manual to Org!  (I haven't really
gotten around to adjusting magit.org and my other manuals yet though.)

Anyway, I have an only marginally related feature request:

Currently ox-texinfo uses the text before the first Org section as the
"Top" Info node.  That is fine for manuals like org-manual.org where
that part of the document only consists of a few lines.  In my manuals
there are up to 40 lines before the first heading, which makes it
desirable to make that part of the document collapsible.

As a side-note, the reasons why I put more than just

> #+title: The Org Manual
>
> #+texinfo: @insertcopying

at the beginning of the file is that (1) I like to set the variables
there, and (2) I put more information into the Info "Top" node (which is
not included in the pdf export) than what I would like to be displayed
on the copyright page when exporting to pdf.  The latter should only
display the copyright information, while I want the former to display
e.g.:

> Borg User Manual
>
> The Borg assimilate Emacs packages as Git submodules.  Borg is a
> bare-bones package manager for Emacs packages.
>
> This manual is for Borg version 2.0.0 (v2.0.0-59-g89a9394+1).
>
>     Copyright (C) 2016-2018 Jonas Bernoulli <address@hidden>
>     ...

It is already possible to set the section whose content is to be used
on the copyright page (and which can be inserted elsewhere using
"#+texinfo: @insertcopying") using the COPYING property.

Could you please implement something similar for the "Top" node, using
a property such as TOP?

  Cheers,
  Jonas