Prints a list of all registered signals. Including description and plugin name.

def list_signals(self):
        """
        Prints a list of all registered signals. Including description and plugin name.
        """
        print("Signal list")
        print("***********\n")
        for key, signal in self.app.signals.signals.items():
            print("%s (%s)\n  %s\n" % (signal.name, signal.plugin.name, signal.description))