This refactor solves a problem or two, the big one being recursing into
group-shapes to get all shapes on the slide, but mostly lays the
groundwork to allow us to refine further aspects such as list-item
detection, off-slide shape detection, and image-capture going forward.
Reviewers: I recommend reviewing commit-by-commit or just looking at the
final version of `partition/docx.py` as View File.
This refactor solves a few problems but mostly lays the groundwork to
allow us to refine further aspects such as page-break detection,
list-item detection, and moving python-docx internals upstream to that
library so our work doesn't depend on that domain-knowledge.
