Add IrcSession::closeLater()

[jpnurmi]

Inspired by QObject::deleteLater(), consider something like this:

if (session->isConnected()) {
    session->sendCommand(IrcCommand::createQuit(reason));
    connect(session, SIGNAL(disconnected()), session, SLOT(deleteLater()));
    connect(session, SIGNAL(socketError(QAbstractSocket::SocketError)), session, SLOT(deleteLater()));
    QTimer::singleShot(timeout, session, SLOT(deleteLater()));
} else {
    session->deleteLater();
}

Open questions:

• should the quit-command be left to the user, or pass the quit reason as a parameter?
• specify the timeout as an optional parameter?

[jpnurmi]

C++:

session->quit(reason);
QTimer::singleShot(timeout, session, SLOT(deleteLater()));

QML:

session.quit(reason);
session.destroy(timeout);

[jpnurmi]

Document the recommended approach in IrcConnection::close() docs.