Add parbox backend

[bitzl]

At the moment cells are created using minipages. As a consequence, footnotes will be set inside the cell. Using parbox as a backend would enable footnotes at the bottom of the page.

Therefore, there should be options to switch to parbox as new backend:

Use parbox as default backend:

\usepackage[backend=parbox]{grid-system}

Use parbox as backend for a row:

\begin[backend=parbox]{row}
    \begin{cell}
        ...
    \end{cell}
\end{row}

Use parbox as backend for one cell:

\begin[backend=parbox]{cell}
    ...
\end{cell}

This will require architecture changes to enable custom backends.