registrar: add templatefilter to mark a function as template filter (API) This patch also adds loadfilter() to templatefilters, because this combination helps to figure out how they cooperate with each other. Listing up loadfilter() in dispatch.extraloaders causes implicit loading template filter functions at loading (3rd party) extension. This change requires that "templatefilter" attribute of (3rd party) extension is registrar.templatefilter or so.