As long as the responder uses only text-single, list-single and
boolean fields, it is possible to invoke and complete a command.
Other field types are not implemented now in dataforms.py.
Introduced a widget that will present full data form as in JEP-0004, to make
things easier. Code will just have to create this widget with just-got
xmpp.DataForm form, show it to user, wait a bit and send the modified form
back.