Select thumbnail next to the currently selected one.

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