Dialogue server now accepts XML format messages

Another of those things I should no doubt have done years ago: after a discussion with Maria, I've now modified the dialogue server so that it can also run in a mode where all messages are XML-formatted.

Details (this is documented in $REGULUS/Prolog/dialogue_server.pl):

- Initially, the server is in Prolog mode.

- To put the server into XML mode, send the message

xml_messages.

- Subsequent messages are of the form

xml_message(XMLMessage).

where XMLMessage is an XML encoding of the corresponding Prolog message produced using the predicate prolog_xml/2 in $REGULUS/PrologLib/prolog_xml.pl.
The XML can be converted back into Prolog if necessary using the same predicate.

I have tested by converting the CALL-SLT Prolog client to use XML-flavor messages, and  it all works fine.

The routines in $REGULUS/PrologLib/prolog_xml.pl should in general be useful for translating Prolog into XML form in a reversible way. Look at the file for documentation and an example.