Using rst-budoux lib and revealjs-break directive, second section' title is broken

[attakei]

Reproduction

• Use demo
• Install and register rst-budoux
• Unregister sphinnxcontrib-budoux
• Run make -C demo demo
• Run localserver
• Access http://localhost:8000/ja/#/6/3

Expected

Keep 「コードのハイライト」 on title.

Actually

Title is 「コードの」 only (「ハイライト」 is dropped)

[attakei]

Reason

In v3.0.1, writer append refer title section from parent, but first node only.

ref:

sphinx-revealjs/sphinx_revealjs/writers.py

Line 154 in 977b479

self.body.append(title.children[0])

Therefore, this issue is reproduced by only sphinx-revealjs

**Strong** title
=============

.. revealjs-break::

[attakei]

Currently, I am fixing issue by feature/transform-doctree branch.

This is not only simple bug fix, includes restructure behavior in generating html.