Individually, these don't make much difference, but they add up
to a halving the time to save on my machine.

EmbeddedObjectContainer shows a lot of work in HasEmbeddedObject and
GetEmbeddedObjectName, so add a reverse map there.

Change-Id: Ib758668dbb045e6ceb2611bd86aa2af4fbfb9917
Reviewed-on: https://gerrit.libreoffice.org/70309
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>