Apps and widgets can add a `<appid>.settings.js` file which can be
opened from the "App/widget settings" submenu.
This file should define a single function to configure the app/widget.
The function is passed a `back` argument, which can be used to return to
the settings menu by calling `back()`.
Example `<appid>.settings.js`:
```
function settings(back) {
const mySettingsMenu = {
'': { 'title': 'My Widget' },
'< Back': back, // go back to settings menu
'Sound': {
value: false,
format: s => s ? 'on' : 'off',
onchange: s => {if(s) Bangle.beep()} // TODO: save new value
},
};
E.showMenu(mySettingsMenu)
}
```