Add IrcChannelModel::add/removeChannel(title)

This opens a whole new interesting opportunity to (ab)use IrcChannel
objects for private queries... :D If this hack turns out to work well,
then the question is... should IrcChannel(Model) actually have a more
generic name, such as IrcBuffer(Model).
Rename IrcChannel/Model to IrcBuffer/Model
Example client: add support for private queries
Remove IrcUser::messageReceived()

It was too ambiguous and I can't think of any real use cases
anyway, so it's better just to remove it for now...
Fix IrcBuffer rename
Add IrcBuffer::type()
IrcBufferModel: create query buffers automatically
Replace IrcBuffer::session() with IrcBuffer::model()

The session is accessible via the model.
Add IrcUser::buffer()
Follow buffer name changes