Reset Section Heading placeholder to blank after Chapter Heading

Currently, the Section Heading placeholder does not reset after a new Chapter Heading. This means that if a template is set up with these placeholders in the footer like:

Chapter Heading placeholder / Section heading placeholder

the generated export can include a section heading from a previous chapter if the new chapter doesn't contain a new section heading (level 2).

For instance, when exporting pages that contain Scroll (Heading level 1), then PDF (heading level 2), then on a different page, have Exporter (Heading level 1), after exporting with the formatting described above the the export will output:

Page 1 Footer: Scroll / PDF

Page 2 Footer: Exporter / PDF

In this case page 2 is incorrect as PDF refers to the previous chapter and is not included in the new Exporter chapter.