Merging ox-texinfo+ into ox-texinfo

[Jonas Bernoulli <jonas@bernoul.li>]

Hello,

In the olden days before Org's own manual was written using an org-mode
file, I started doing just that for Magit.  Because ox-texinfo.el wasn't
quite there yet I wrote an extension, ox-texinfo+.el, to fill in the
gaps.  Since then I have written seven more manuals that use my
extension.

I recently talked to Bastien about this and he encouraged to bring up
the possibility of merging ox-texinfo+.el into ox-texinfo.el.

ox-texinfo+ (https://github.com/tarsius/ox-texinfo-plus) has several
features but the one I would like to talk about now is the following.

[If you want to look at the other features now, then please use the
 "next" branch as I am in the process of trimming some of them down
 or even removing them completely.  The main feature is the same in
 the "master" and "next" branches.]

   Create `@deffn` and similar definition items by writing list
   items in Org that look similar to what they will look like in
   Info.  To enable this, add:

   #+TEXINFO_DEFFN: t

   to your Org file.  After doing that, you can create definition
   items like so:

   - Command: magit-section-show

     Show the body of the current section.

   - Function: magit-git-exit-code &rest args
   - Macro: magit-insert-section &rest args
   - Variable: magit-display-buffer-noselect
   - User Option: magit-display-buffer-function
   - Key: q, magit-mode-bury-buffer

I propose that we add this as an optional feature to ox-texinfo.el
itself.

IMO the biggest advantage of this style is that it leads to a prettier
org file, which is suitable for direct consumption by end-users.  It is
also easier to write in this style by package authors who might not want
to fully familiarize themselves with all the peculiarities of writing an
org file intended for export to texinfo.

It is possible to mix the two styles; you can use the ox-texinfo+.el
style for most or all definitions but use the additional flexibility of
ox-texinfo.el, when that is needed.

What do you think?  Might this be something that could be merged?

The reason I am bringing this up now after years of maintaining this as
an extension is that I would like to finally stop checking in both the
*.org and *.texi files into git.  [Non]GNU Elpa already supports this
and I plan to implement it for Melpa as well.  It would be much nicer if
the only additional requirement for an *elpa was to have a recent enough
Org version installed, instead of that and also ox-texinfo+.el.

     Cheers,
     Jonas