Fixup wendelin gadet data stream ui

1. portal_gadgets/WendelinInformationGadget state should be made public (not invisible like now)

2. portal_gadgets/WendelinInformationGadget should have a default thumbnail (i.e. image). You can simply make print screen and add it

3. When Wendelin gadget rendered I see links like http://wendelin:2200/erp5/$%7Bportal_url%7D/portal_skins/erp5_data_notebook/manage_main looks like script not called that replaces it dynamically ?

4. In /DataStream_view my_append_file is in group "not_assigned" but it should be right under "my_file" in "right" groupd

@Tyagov: everything fixed as requested.