Remove cyclical dependence of SectionHeading component

[Dananji]

Description

#643 (review)

Not in scope of this PR but when looking through it I thought the cyclical dependence of SectionHeading on List and List on SectionHeading is confusing. I wonder if having an abstract component with subclasses might help scope and conceptualize the different uses of the two components.

Done Looks Like

• Implement an abstract component to remove cyclical dependence of SectionHeading component when populating structure
• Test structure is rendered as expected for all use-cases;
  • Avalon media object manifests (with structure and w/o structure)
  • Avalon playlist manifest
  • Cookbook table of contents manifests
    • https://iiif.io/api/cookbook/recipe/0026-toc-opera/
    • https://iiif.io/api/cookbook/recipe/0064-opera-one-canvas/
    • https://iiif.io/api/cookbook/recipe/0065-opera-multiple-canvases/