Do not use dropdowns for items with depth set to <= 1

If the depth is set to 1 for a multi-level menu, the built dropdown ends up unusable as it lacks any children (as expected) and it's uri is set to ``#``.

```
{{ mopa_bootstrap_menu('foo', { depth: 1 }) }}
```

This PR introduces an check for the depth option in addition to the existing check of ``$item->hasChildren()``.