Scroll display down one line.

def scroll_down (self): # <ESC>D
        '''Scroll display down one line.'''

        # Screen is indexed from 1, but arrays are indexed from 0.
        s = self.scroll_row_start - 1
        e = self.scroll_row_end - 1
        self.w[s+1:e+1] = copy.deepcopy(self.w[s:e])