\centering doesn't work in plainframes

This doesn't center the text:

``` latex
\begin{plainframe}{}
\centering
bla bla bla\ldots
\end{plainframe}
```

On the other hand, this does:

``` latex
\begin{frame}{}
\centering
bla bla bla\ldots
\end{frame}
```

I guess we should find out why this happens, or at least document it... :wink:

(Note that `\begin{center} ... \end{center}` works in both environments, so it is not a huge issue :smile:)

Compare how the plainframe and frame environments handle \centering, using the examples in the issue as the reproduction case. Determine whether plainframe should support the same behavior or whether the limitation needs documentation; done means the alignment works consistently or the behavior is clearly documented.

Written by the indexing model from the issue text.