Bug 1054274 - Split out browser overflow menu into shared component. r=benfrancis

Seems fragile to used the `data-l10n-id` attribute as a hook here. If/when the string changes, the id will be changed and then this JS will break. I always prefer to have a dedicated class, purely for use as a JS hook.

```html
<li class="foo js-foo"></li>
```

```js
var foo = document.querySelector('.js-foo');
```
That way styling classes, tags, context, etc can change; and markup editors know to stay clear of the `js-foo` selector.