Add IrcChannelModel::titles() [property]
Declare IrcChannel & IrcUser as meta types in the headers

This way the user doesn't have to do it, but these work out of the box:
- QVariant::value<IrcChannel*>() for Irc::ChannelRole, and
- QVariant::value<IrcUser*>() for Irc::UserRole.
Make the example client capable of joining multiple channels