Redo the section layout

[gregturn]

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• Fix the Packt style used
• Pull out chapter number as a section and instead rely on attribute :chapter_number, defaulting to 1 if not specified
• Move everything up, so that main sections are Level 1, i.e. ==, etc.
• Add a preamble

Issues:

• Can't seem to grab the title inside the preamble. docbook's block_preamble.xml.slim seems to use it. Why can't I?
• If the Chapter Title styling works, do I need to then rip it's relic out of section.fodt.slim?