DOCX import: set document-level font size default based on default para style

DOCX has two defaults: there can be default paragraph/run properties,
and also a paragraph/character style can be marked as default.

In this case the problem was that no doc-level default was specified,
but the default para style set a char height, and shape text is expected
to inherit that. Fix this by setting doc-level font size to default para
style, in case it's not set yet -- that matches what the binary import
does.

Also, adjust the export side, so that these duplicated default font size
is not written on export.

Change-Id: I63b368e7704142171d58f48d08052ac7616ab166