Resume enumerate over frames [updated]

Originally reported by: Anonymous

Hey ya,

this is a duplicate of #62, but I saw the option to insert code too late. That's why I opened a new one with the correct commands to insert code.

I tried to resume the numbering over two and more frames.

I tried the solution offered here
(http://www.latex-community.org/forum/viewtopic.php?f=44&t=9362&start=10#), but
this changes the nice retangle enumerates to "normal" looking numbers, ... and
is more a hack than a solution.

I would be nice to have a solution within the beamer-class which resumes the
numbers over any number of frames.

\begin{frame}{title}{subtitle I}
    \begin{itemize}[<+->]

        \item First item
            \includegraphics[scale=.75]{1.png}
        \item Second item
            \includegraphics[scale=.75]{2.png}
        \item Third item
            \includegraphics[scale=.75]{3.png}
    \end{itemize}

\end{frame}
\begin{frame}{title}{subtitle II}
    \begin{itemize}[<+->][resume]

        \item Fourth item
            \includegraphics[scale=.75]{1.png}
        \item Fifth item
            \includegraphics[scale=.75]{2.png}
        \item Sixth item
            \includegraphics[scale=.75]{3.png}
    \end{itemize}

\end{frame}

A small ASCII-graphic to illustrate my feature request.

Frame 1
---------------------
title
  subtitle I
========================


         1. First item
         -------------
        |             |
        |             |
        |             |
         -------------

         2. Second item
         -------------
        |             |
        |             |
        |             |
         -------------

         3. Third item
         -------------
        |             |
        |             |
        |             |
         -------------

Frame 2
---------------------
title
  subtitle II
========================

         4. Fourth item
         -------------
        |             |
        |             |
        |             |
         -------------
         5. Fifth item
         -------------
        |             |
        |             |
        |             |
         -------------
         6. Sixth item
         -------------
        |             |
        |             |
        |             |
         -------------

• Bitbucket: https://bitbucket.org/rivanvx/beamer/issue/63