get_actor --> get_name_from_jid so it is visible it is on gajim level and not on xmppy (lower) lever

custom appications vbox only shows up when we select Custom. this cleans up UI by making it more robust and less confusing and gains  in height of contents of advanced tab in prefs window

enrich with info on how to ban domain and what else is allowed; make Close --> Cancel and Apply --> OK; FIXME: OK cb is not called.. :(