Add QMetaMethod::fromSignal() function

Add QMetaMethod::fromSignal() function

Given a member function that's a signal, returns the corresponding
QMetaMethod. Inspired by the implementation of the template-based
QObject::connect().

The primary use case for this function is to have an effective and
exact (not subject to shadowing) way of checking whether a known
signal was connected to in reimplementations of
QObject::connectNotify(QMetaMethod), avoiding string comparisons.
Example:

void MyObject::connectNotify(const QMetaMethod &signal)
{
    if (signal == QMetaMethod::fromSignal(&MyObject::mySignal)) {
        // Someone connected to mySignal ...
    }
}

Change-Id: I5e4de434275fe543c004d569dcaa9ceda3442f03
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>