Support symfony 2.4 form profiler panel

When using Symfony 2.4 and using FormServiceProvider the form profiler panel is added.

The template is registered in any case, as the TemplateManager only tries to load the template if a collector is registered with this name.

**Requires** silexphp/Silex#880 to be merged in order to get this working. It will not break if merged before, but will not be activate.