Terminal: Save settings to config file

When exiting the terminal settings window, the opacity, bell and
maximum history size will be saved to the application config file.
Because the color scheme is neither configurable nor
will it be loaded on startup, it will not get saved for now.

Userland/Applications/Terminal/main.cpp

@@ -357,6 +357,23 @@ int main(int argc, char** argv)
                 settings_window = create_settings_window(terminal);
             settings_window->show();
             settings_window->move_to_front();
+            settings_window->on_close = [&]() {
+                config->write_num_entry("Window", "Opacity", terminal.opacity());
+                config->write_num_entry("Terminal", "MaxHistorySize", terminal.max_history_size());
+
+                auto bell = terminal.bell_mode();
+                auto bell_setting = String::empty();
+                if (bell == VT::TerminalWidget::BellMode::AudibleBeep) {
+                    bell_setting = "AudibleBeep";
+                } else if (bell == VT::TerminalWidget::BellMode::Disabled) {
+                    bell_setting = "Disabled";
+                } else {
+                    bell_setting = "Visible";
+                }
+                config->write_entry("Window", "Bell", bell_setting);
+
+                config->sync();
+            };
         });
 
     terminal.context_menu().add_separator();