class INotifyer {
   virtual ~INotifyer() {}

   virtual void someSignal() = 0;
   virtual void someSlot() = 0;
};
class Notifyer : public QObject, public INotifyer {
   Q_OBJECT

signals:
   virtual void someSignal() = 0;

public slots:
   virtual void someSlot() = 0;
};
class IPlugin {
   virtual ~IPlugin() {}
   virtual INotifyer* notifyer() = 0;
};
class Plugin : public IPlugin {
   Q_OBJECT
   Q_INTERFACES(IPlugin)
public:
   INotifyer* notifyer() {
      if (!m_Notifyer)
         m_Notifyer = new Notifyer();
      return m_Notifyer;
   }
private:
   Notifyer *m_Notifyer;
};

emit m_Notifyer->someSignal()

connect(plugin->notifyer(), SIGNAL(someSignal()), this, SLOT(applicationSlot()));

virtual QObject *instance() = 0;

virtual QObject *instance() { return this; }

connect(plugin->notifyer()->instance(), SIGNAL(someSignal()), this, SLOT(applicationSlot()));