Avoid string-based lookup in IS_SIGNAL_CONNECTED

Specify the signal as a member function suitable for passing to
QMetaMethod::fromSignal(), and use the new function
QMetaObjectPrivate::signalIndex(QMetaMethod) to get the index in the
signal range.

Change-Id: If16daa24c2699f7749a17decb611cf395d89d0c4
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>