give warning if save folder doesn't have write permission

When developing tabs for tps, when never this error happens, the tps figure is cleared which isn't good for user experiment.

To fix it, add code to check before close SavePlotDialog

        if not os.access(save_file_path, os.W_OK):
            QtWidgets.QMessageBox.information(
                self, "No Write Permission",
                "The directory to save file to doesn't have Write Permission."
                "\nPlease change its permission or "
                "select another directory."
            )
            return