#27 review and test list widget

[Ivan Bogicevic]

The list widget is buggy.
1. Review all implementation parts of the list widget.
2. Test the list with
- all kinds of widgets types inside the list
- with 0, 1, and multiple property types inside the list
- changing the list structure after data for a module has been filled out, e. g. deleting structure element and adding some new ones