Assemble the current components in the QStimulusModel and generate a spectrogram 
        plot in a separate window

def preview(self):
        """Assemble the current components in the QStimulusModel and generate a spectrogram 
        plot in a separate window"""
        msg = self.ui.trackview.model().verify()
        if msg:
            answer = QtGui.QMessageBox.warning(self, "Bummer", 'Problem: {}.'.format(msg))
            return
        stim_signal, atten, ovld = self.ui.trackview.model().signal()
        fig = SpecWidget()
        fig.setWindowModality(2) # application modal
        fig.updateData(stim_signal, self.ui.trackview.model().samplerate())
        fig.setTitle('Stimulus Preview')
        fig.show()
        self.previewFig = fig