Remove all thumbnails.

def remove_all_thumbnails(self):
        """Remove all thumbnails."""
        for thumbnail in self._thumbnails:
            self.layout().removeWidget(thumbnail)
            thumbnail.sig_canvas_clicked.disconnect()
            thumbnail.sig_remove_figure.disconnect()
            thumbnail.sig_save_figure.disconnect()
            thumbnail.deleteLater()
        self._thumbnails = []
        self.current_thumbnail = None
        self.figure_viewer.figcanvas.clear_canvas()