Select the thumbnail previous to the currently selected one.

def go_previous_thumbnail(self):
        """Select the thumbnail previous to the currently selected one."""
        if self.current_thumbnail is not None:
            index = self._thumbnails.index(self.current_thumbnail) - 1
            index = index if index >= 0 else len(self._thumbnails) - 1
            self.set_current_index(index)
            self.scroll_to_item(index)