fdo#80410: sw: DOCX export: support DOCX embedded objects

(cherry picked from commit 3ffb06e76be9be7499f12973139949ddbf6297b9)
fdo#80410: let's add a unit test too...
(cherry picked from commit 46d9724369ba4f248eab0133d396bd8f6a4453c6)

Change-Id: Ifa9e12eee4139a9c9c5d17a349ff3511c62b3ea8
Reviewed-on: https://gerrit.libreoffice.org/10234
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>