Added propertyaccessor based multisort

This PR makes two things possible:

- Sorting ANY collection in a Twig template
- Accessing the system property `order` from query results (e.g. smart content results)

````jinja
      {% for page in content.smartcontent|sulu_util_multisort('[order]', 'asc') %}

          <div class="col-lg-4">
              <h2>
                  <a href="{{ content_path(page.url) }}">{{ page.title }}</a>
              </h2>
              <!-- ... -->
      {% endfor %}
````

The twig function `sulu_util_multisort` sorts arrays or objects, on properties or methods thanks to the [PropertyAcessor](http://symfony.com/doc/current/components/property_access/introduction.html).

The twig extension uses `SortUtil::multisort` function internally

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | n/a
| BC Breaks     | n/a
| Doc           | 

@danrot decided to go with `order`, as it is indeed more important to be consistent here I think, even if everything else is suboptimal.