The previous built-in reference doc had only title and content
layouts. Add in a section-header slide and a two-content slide, so
users can more easily modify it to build their own templates.
Golden files needed to be regenerated. Checked on MS PowerPoint 2013.
* docx writer: support custom properties. Solves the writer part of #3024.
Also supports additional core properties: `subject`, `lang`, `category`,
`description`.
* odt writer: improve standard properties, including the following core properties:
`generator` (Pandoc/VERSION), `description`, `subject`, `keywords`,
`initial-creator` (from authors), `creation-date` (actual creation date).
Also fix date.
* pptx writer: support custom properties. Also supports additional core
properties: `subject`, `category`, `description`.
* Includes golden tests.
* MANUAL: document metadata support for docx, odt, pptx writers
Make sure there are no empty slides in the pptx output. Because of the
way that slides were split, these could be accidentally produced by
comments after images.
When animations are added, there will be a way to add an empty slide
with either incremental lists or pauses.
Test outputs checked with MS PowerPoint (Office 2013, Windows 10,
VBox). Both files have expected output and are not corrupted.
