Made TabbedView title customizable by a custom tabbedview template.

In some cases the TabbedView title should be customizable, for example when you need a dynamic value.

With this changes it's easy to overwrite the title value in a simple custom template:
```
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"
      xmlns:tal="http://xml.zope.org/namespaces/tal"
      xmlns:metal="http://xml.zope.org/namespaces/metal"
      xmlns:i18n="http://xml.zope.org/namespaces/i18n"
      lang="en"
      metal:use-macro="context/@@tabbed_view/macros/master"
      i18n:domain="ftw.tabbedview">

  <metal:title metal:fill-slot="content-title">
    <h1 class="documentFirstHeading" tal:content="view/dynamic_title">
    </h1>
  </metal:title>

</html>
```

Used by https://github.com/4teamwork/opengever.core/issues/284

\cc @maethu, @jone