Compared to the ClearSilver version, there's one additional parameter for the Genshi version: instead of `template` and `content_type`, it's now `template`, `data` and `content_type`. This makes it convenient to support both the 0.10 and 0.11 request filters, as we can distinguish them based on the arity of the `post_process_request` method.
The main usage for such a post-process filter is to change the data dictionary (aka template context) before the template is rendered.
The Timeline makes use of this to replace the `dateinfo` method, so that all the dateinfos become links to the timeline. Thanks to you-know-who for the clever idea ;)
Implements #975.
