[Pimcore] introduce twig placeholder

[dpfaffenbauer]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no

Allows that in Mail Documents:

%Twig(files, {'template' : ':Mail/includes:files.html.twig'});

The corresponding template looks like that:

<ul>
    {% for file in files if file is asset %}
        <li><a href="{{ file }}">{{ file.getFilename }}</a></li>
    {% endfor %}
</ul>