Advanced Diagram support: cleanup/consolidate with existing code

Reorganized and streamlined, use IDiagramHelper as main interface
now also for existing code.

Had to adapt oox::Shape && Diagram handling since there the
import gets handled very different. This ensures that a Diagram
is detected at export and that the same happens for now as before

Had to add a detection that resetting the GrabBag is meant to
disable the Diagam functionality. That is very indirect, but
has to stay for compaibility reasons for now

Change-Id: I620b7d61cd84b5f9dd8ae4dc890ebf70ce779cdf
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/130389
Tested-by: Jenkins
Reviewed-by: Armin Le Grand <Armin.Le.Grand@me.com>

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