[Engine-XMPP] CS: don't make fields public that the frontend never needs

oliver@oliver-laptop:~/Projects/smuxi$ git log -p src/Frontend-GNOME-XMPP/ | grep OwnMessage
oliver@oliver-laptop:~/Projects/smuxi$ git log -p src/Frontend-GNOME-XMPP/ | grep SeenNewMessages
oliver@oliver-laptop:~/Projects/smuxi$ git log -p src/Frontend-GNOME-XMPP/ | grep LastSeenStamp
oliver@oliver-laptop:~/Projects/smuxi$ git log -p src/Frontend-GNOME-XMPP/ | grep Password

looks fine to me