Register shell with figure explorer.

        This function opens a new FigureBrowser for browsing the figures
        in the shell.

def add_shellwidget(self, shellwidget):
        """
        Register shell with figure explorer.

        This function opens a new FigureBrowser for browsing the figures
        in the shell.
        """
        shellwidget_id = id(shellwidget)
        if shellwidget_id not in self.shellwidgets:
            self.options_button.setVisible(True)
            fig_browser = FigureBrowser(
                self, options_button=self.options_button,
                background_color=MAIN_BG_COLOR)
            fig_browser.set_shellwidget(shellwidget)
            fig_browser.setup(**self.get_settings())
            fig_browser.sig_option_changed.connect(
                self.sig_option_changed.emit)
            fig_browser.thumbnails_sb.redirect_stdio.connect(
                self.main.redirect_internalshell_stdio)
            self.register_widget_shortcuts(fig_browser)
            self.add_widget(fig_browser)
            self.shellwidgets[shellwidget_id] = fig_browser
            self.set_shellwidget_from_id(shellwidget_id)
            return fig_browser