Implement secure channel / OTR support

[diederik]

For encrypted chat, the official client has a "SChannel" flag. This will probably encrypt messages with SSL/TLS. There is a 'schannel' capability flag, and a "SChannel: 0" field in P2P/MSNSLP invitations.

Since not every client supports this, the user interface must give proper feedback about this.

Some third party clients (e.g. Kopete in KDE 4.1) have their own mechanism already (called "Off the record" support). It would be useful to support this as well, to increase the chance of having secure conversations.

See:

• http://www.kmess.org/board/viewtopic.php?f=6&t=3406
•  http://en.wikipedia.org/wiki/Off-the-Record_Messaging
•  http://lists.kde.org/?t=120397998900007&r=1&w=2
•  http://kopete-otr.follefuder.org/news.html

[diederik]

Another request for OTR: http://www.kmess.org/board/viewtopic.php?f=2&t=3841