Beamer export of columns should use \columnwidth

[Julien Cubizolles <j.cubizolles@free.fr>]

For the moment, the :BEAMER_COL: property of a heading is exported as a
\textwidth argument to the column environment of beamer like:

--8<---------------cut here---------------start------------->8---
\begin{columns}
\begin{column}{0.5\textwidth}
\end{column}
\end{columns}
--8<---------------cut here---------------end--------------->8---

That's fine for a standard beamer file but I'm also using the beamer
export backend to produce some TeX files using my personal class with
the beamerarticle package. My class uses a 2 column setup for wich the
\textwidth spans the two columns. The right length should be
\columnwidth, as in:

--8<---------------cut here---------------start------------->8---
\begin{columns}
\begin{column}{0.5\columnwidth}
\end{column}
\end{columns}
--8<---------------cut here---------------end--------------->8---

I don't think there would be any inconvenients to changing \textwidth to
\columnwidth for regular beamer files (it's fine with my beamer files so
far).

Julien.