fix(docx): tables in header/footer dropped (#2135)
A DOCX header or footer is a so-called "story part" meaning like the
document body (which is also a story part) it can contain both
paragraphs and tables. The implementation of `Header.text` and
`Footer.text` gather only the paragraphs.
Add a new method to extract all content from a header or footer,
including table content, suitable for use as the `.text` attribute of
that element.
Fixes #2126.
