changeset_templater: backport parts map of [templates] section from formatter

formatter: add support for parts map of [templates] section Unlike a mapfile whose template is looked up by spec -> mapfile -> topic, [templates] section is global. We use :sub-section syntax to define parts per template.

changeset_templater: backport separator template from formatter As commented, this should be used with docheader and docfooter, not with header nor footer. That's one reason why no props are passed to templater when rendering a separator. (See map-cmdline.changelog to understand what the "header" is.)