Add plugable backends

[bitzl]

All cells are created using minipages. To enable switching to other methods, like parbox, we need to enable pluggable backends.

This will change the architecture to something like:

\definegridbackend{minipage}[1]{\begin{minipage}{#1}}{\end{minipage}}

\begin{backend}{<thebackend>}{<width>}{<options>}
\end{backend}