[Dune] Update of the official Homepage / dunedesign.png

[Andreas Dedner]

That is similar to what I wanted to have with my last figure
(ok it did not use the L-shape domain and I wanted to show that
grid modules are available - misunderstanding the dots).
So I wanted to have something like:

> +--------------[app]-[app]-[app]---------------+
> |                                              |
> +-----------------+                            |
> | extmod1    ...  |                            |
> +          +------+---------+                  |
> | extmod2  | pdelab   fem   |                  |
> +----------+------+---------+-[grids]-[grids]--+
> | common     grid     istl    localfunctions   |
> +----------+------+---------+------------------+