Register shell with variable explorer.

        This function opens a new NamespaceBrowser for browsing the variables
        in the shell.

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

        This function opens a new NamespaceBrowser for browsing the variables
        in the shell.
        """
        shellwidget_id = id(shellwidget)
        if shellwidget_id not in self.shellwidgets:
            self.options_button.setVisible(True)
            nsb = NamespaceBrowser(self, options_button=self.options_button)
            nsb.set_shellwidget(shellwidget)
            nsb.setup(**self.get_settings())
            nsb.sig_option_changed.connect(self.change_option)
            nsb.sig_free_memory.connect(self.free_memory)
            self.add_widget(nsb)
            self.shellwidgets[shellwidget_id] = nsb
            self.set_shellwidget_from_id(shellwidget_id)
            return nsb