Unlike Qt5, the Qt4 signals implementation doesn't allow a signal to be
directly connected to a c++ lambda expression. Work around this by defining a
Callback QObject with a virtual method that can forward calls to a closure.
The Qt4 error was reported by Patrick Strateman <patrick.strateman@gmail.com>
in https://github.com/bitcoin/bitcoin/pull/10039#issuecomment-289248763
