QObject::disconnect with new syntax

QObject::disconnect with new syntax

This add an overload to disconnect which is symetrical to the new
syntax of connect.

It is possible to diconnect connection like this:

QObject::connect( sender, &Sender::valueChanged,
                  receiver, &Receiver::updateValue );
QObject::disconnect( sender, &Sender::valueChanged,
                     receiver, &Receiver::updateValue );

This overload only work with pointer to member function, and not static
functions or functors.

The test is copied from tst_QObject::disconnect(), just
changed the syntax of the connection and disconnection

Change-Id: Ia8f819100cb12098e32877522b97b732b1e676a8
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>