struct2str recursive

Displays a struct's content in a list box. View a substruct's content by selecting its list entry.

A small extension of struct2str by Marco Cococcioni:
- Selecting a substruct in the listbox results in calling struct2str again on the chosen substruct.
- Multidimensional struct-matrices are now completely displayed by making the input struct one-dimensional.

See struct2str for a detailed description.