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

Unless you absolutely need another element, I would try to stick to just one per component. Then your markup could look something like:

```html
<gaia-overflow-menu>
  <li>Option 1</li>
  <li>Option 2</li>
  <li>Option 3</li>
</gaia-overflow-menu>
```

This also gives the freedom for the user to use `<a>`s or `<button>`s:

```html
<gaia-overflow-menu>
  <button>Option 1</button>
  <button>Option 2</button>
  <a href="#somewhere">Option 3</a>
</gaia-overflow-menu>
```

Obviously for some more complex components, sub-components will be required, but in this case seems like we can get away without.