registrar: add templatefunc to mark a function as template function (API) This patch also adds loadfunction() to templater, because this combination helps to figure out how they cooperate with each other. Listing up loadfunction() in dispatch.extraloaders causes implicit loading template function at loading (3rd party) extension. This patch explicitly tests whether templatefunc decorator works as expected, because there is no bundled extension, which defines template function. This change requires that "templatefunc" attribute of (3rd party) extension is registrar.templatefunc or so.