Expose is_current_parent in menu template

This property allow template to know whether or not the page we're looping over is the parent of the current page.

My case is the following:

* If the current page is at the top level, I display:

```
Title: Current page
list of sub pages
```

* If the current page is not at the top level, I display:

```
Title: Parent page
list of sub pages
```

Also, I'm doing some page preview, and the slug is not in the URL. So `page.is_current` doesn't work. Why not use the pk instead? It doesn't break any test.

Here's how I'm using `page.is_current_parent`:
```html
{% load pages_tags %}

{% if page_branch_in_menu %}
  {% for page in page_branch %}
    {% if page.is_primary and page.is_current %}
      <h1 class="here">{{ page.title }}</h1>
      <ul>
        {% page_menu page %}
      </ul>
    {% elif page.is_current_or_ascendant and not page.is_current %}
      {% if page.is_current_parent %}
        <h1><a href="{{ page.get_absolute_url }}">{{ page.title }}</a></h1>
        <ul>
      {% endif %}
      {% page_menu page %}
      {% if page.is_current_parent %}
        </ul>
      {% endif %}
    {% elif not page.is_primary %}
      {% if page.is_current or page.is_current_sibling or page.is_current_child %}
      <li{% if page.is_current %} class="here"{% endif %}><a href="{{ page.get_absolute_url }}">{{ page.title }}</a></li>
      {% endif %}
    {% endif %}
  {% endfor %}
{% endif %}
```