Add a documentation for Tp::BaseDebug class

[Kaffeine]

Besides of the documentation, it would be good to also add an inline example, such as:

#include <TelepathyQt/BaseDebug>

static QPointer<Tp::BaseDebug> debugInterfacePtr;
static QtMessageHandler defaultMessageHandler = nullptr;

void telepathyDebugHandler(QtMsgType type, const QMessageLogContext &context,
                           const QString &msg)
{
    if (!debugInterfacePtr.isNull()) {
        const QString domain = QStringLiteral("%1:%2, %3")
                .arg(QString::fromLatin1(context.file))
                .arg(context.line)
                .arg(QString::fromLatin1(context.function));

        switch (type) {
        case QtDebugMsg:
            debugInterfacePtr->newDebugMessage(domain, Tp::DebugLevelDebug, msg);
            break;
        case QtInfoMsg:
            debugInterfacePtr->newDebugMessage(domain, Tp::DebugLevelInfo, msg);
            break;
        case QtWarningMsg:
            debugInterfacePtr->newDebugMessage(domain, Tp::DebugLevelWarning, msg);
            break;
        case QtCriticalMsg:
            debugInterfacePtr->newDebugMessage(domain, Tp::DebugLevelCritical, msg);
            break;
        case QtFatalMsg:
            debugInterfacePtr->newDebugMessage(domain, Tp::DebugLevelError, msg);
            break;
        }
    }

    if (defaultMessageHandler) {
        defaultMessageHandler(type, context, msg);
        return;
    }

    const QString logMessage = qFormatLogMessage(type, context, msg);

    if (logMessage.isNull()) {
        return;
    }

    fprintf(stderr, "%s\n", logMessage.toLocal8Bit().constData());
    fflush(stderr);
}

bool enableDebugInterface()
{
    if (!debugInterfacePtr.isNull()) {
        return debugInterfacePtr->isRegistered();
    }

    debugInterfacePtr = new Tp::BaseDebug();
    debugInterfacePtr->setGetMessagesLimit(-1);

    if (!debugInterfacePtr->registerObject(TP_QT_CONNECTION_MANAGER_BUS_NAME_BASE
                                           + QLatin1String("example"))) {
        return false;
    }

    defaultMessageHandler = qInstallMessageHandler(telepathyDebugHandler);
    return true;
}

void disableDebugInterface()
{
    if (debugInterfacePtr.isNull()) {
        return;
    }

    qInstallMessageHandler(defaultMessageHandler);
    debugInterfacePtr->deleteLater();
    debugInterfacePtr.clear();
    defaultMessageHandler = nullptr;
}