In-buffer completion for macro/env arguments

[Arash Esbati]

Hi all,

I've installed a change (35ec84ba6f) which brings in-buffer completion
for macro and environment arguments to AUCTeX.  This feature relies
solely on the information it finds in `TeX-symbol-list' and
`LaTeX-environment-list' which means that currently:

 1. Wrong completion candidates are expected and/or it doesn't work
    everywhere

 2. Completion in the arguments of \documentclass and \usepackage do not
    work

Making 1. work means changing a lot of style files and bring many
functions defined top-level into the style hooks.  I'm not sure if
that's feasible everywhere, but we have to find out along the way.  For
2., I have currently no idea.

At any rate, I'd appreciate if people could test the current state of
affairs and give some feedback if the code actually works.

Best, Arash