Fix table of contents for headings in tables and collapsible sections

[KatsiarynaDzibrova]

Fixes following issues with table of contents:

• The order in table of contents is wrong for heading inside tables and collapsible sections.
• The heading is duplicated when resizing parent table cell

Now on each heading update dfs is used to get all headings and create a new TOC from scratch. It might take more time, but the change shouldn't be noticeable

Before:

After:

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[KatsiarynaDzibrova]

Made the following changes:

1. Explicitly checking if the heading is already in TOC to avoid duplicates
2. Replaced dfs with listener on root updates, which updates all child headings
3. Find previous heading for a new heading only when it's updated The performance should be much better now as we only visit updated nodes

@zurfyx feel free to take a look