Support the Table of Contents macro on pages listing blog posts

Context

A page can combine the Table of Contents macro with a list of blog posts, for example:

{{toc/}}

{{blogpostlist layout="full"/}}

Problem

The table of contents does not reflect the list of posts well:

• In the Full layout, the date groups are level 2 headings and the post titles level 3 headings, where level 1 and level 2 would make more sense in a table of contents.
• In the Image layout (the default layout of the blog home page) and in the weekly and monthly displays of a blog, the post titles never appear in a table of contents, because these displays are rendered inside an HTML macro.

Expected

The post titles of a blog listing appear in the table of contents, with their text and a stable anchor, and with heading levels suited to a table of contents.