QSignalMapper should use const QObject instead of QObject

QSignalMapper should use "const QObject *" instead of "QObject *", i.e.

QSignalMapper::setMapping( QObject * sender, int id )

should instead be

QSignalMapper::setMapping( const QObject * sender, int id )

as the function does not modify the QObject instance. This goes for the other functions in QSignalMapper taking a "QObject *" as well.