Allow SIGNAL() and SLOT() to be used with QMetaObject::indexOfSignal() and QMetaObject::indexOfSlot()

This is in some way a follow-up to QTBUG-36861.

I too think that generally a complementing `fromSlot` could help, but I think the core issue lies there:

What I find odd is that there is basically no modern way to connect to a signal using its `QMetaMethod`: You have to use the `connect()` overload that takes both the signal and the slot as a QMetaMethod, which is fine by itself, but at the moment I can't find a modern way to get a `QMetaObject` for anything that I could connect to - except for using the string-based `indexOfMethod()` way. That is unfortunate.

It is literally the only place in my code where I still have to use string-based method lookup.

So instead of the already discussed `fromSlot()` which seems to be unavailable by design (moc doesn't report slot offsets?) I think the more viable solution would be to provide the messing overloads for the `connect()` function:

QObject::connect(const QObject *sender, const QMetaMethod &signal, Functor functor);
QObject::connect(const QObject *sender, const QMetaMethod &signal, const QObject *receiver, PointerToMemberFunction method);

However (see comments) that proves to be difficult, as there is no immediate translation between a functor signature and the string based signature, so the connection would immantently loose the (static or runtime) type-safety.

So as a final resort I'd like to propose an addition to the lookup functions to accept signatures provided by the usual `SIGNAL()` and `SLOT()` macros. That would make the usage of signal and slot names consistent between `connect()` and `indexOfSignal()`/`indexOfSlot()`. The addition would be similar to this:

    if (extract_code(slot) == QSLOT_CODE) {
        slot++;
    }
#if QT7
    else {
        qWarning("QMetaObject::indexOfSlot: Use the SLOT or SIGNAL macro to lookup %s::%s",
            priv->mobj->className(), slot);
        return -1;
    }
#endif