preserving SmartArt allows editing it in PowerPoint after saving as pptx file

* moved common parts for docx and pptx export to oox/drawingml
* fixed export tests that expected shapes on output

Change-Id: I3e70a9f4177bebf5e1671232f4cd0ef0e7212626
Reviewed-on: https://gerrit.libreoffice.org/69598
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>