IrcBufferModel::buffer(name) -> find(name)

Reserve buffer() for IrcBuffer* buffer(QModelIndex)
IrcUserModel::user(name) -> find(name)

Reserve user() for IrcUser* user(QModelIndex)
Fix IrcBuffer activity notifications

QWARN  : tst_IrcBufferModel::testAddRemove()
QObject::connect: Cannot connect
    (null)::statusChanged(IrcConnection::Status)
    to IrcBuffer::_irc_emitActiveChanged()
Extend tst_IrcBufferModel