It used to work earlier, but there were an issue with the
shape id and so controls were not find. Also in PPTX import
the persistStorage attribute was handled only for parent
controls and not for other kind of controls.

Change-Id: I9784166b65407b79b6dfed8a38087b55b1b69835
Reviewed-on: https://gerrit.libreoffice.org/40751Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>
Tested-by: Tamás Zolnai <tamas.zolnai@collabora.com>