pptx: Support footers in the reference doc

In PowerPoint, it’s possible to specify footers across all slides,
containing a date (optionally automatically updated to today’s date),
the slide number (optionally starting from a higher number than 1), and
static text. There’s also an option to hide the footer on the title
slide.

Before this commit, none of that footer content was pulled through from
the reference doc: this commit supports all the functionality listed
above.

There is one behaviour which may not be immediately obvious: if the
reference doc specifies a fixed date (i.e. not automatically updating),
and there’s a date specified in the metadata for the document, the
footer date is replaced by the metadata date.

- Include date, slide number, and static footer content from reference
  doc
- Respect “slide number starts from” option
- Respect “Don’t show on title slide” option
- Add tests

0 files changed, 0 insertions, 0 deletions