Updates the interface to show the selection buttons.

def updateUi(self):
        """
        Updates the interface to show the selection buttons.
        """
        index = self._slideshow.currentIndex()
        count = self._slideshow.count()
        
        self._previousButton.setVisible(index != 0)
        self._nextButton.setText('Finish' if index == count - 1 else 'Next')
        self.autoLayout()