PPTX import: fix missing document metadata

Without this, the creation / modification date is lost on import.

(cherry picked from commit ef2668bad976f1fbb70759887cafd35ea7833655)

Conflicts:
	sd/qa/unit/import-tests.cxx

Change-Id: I0b74ac91aee7b8b3e0bc763247086a3a39816bc1
Reviewed-on: https://gerrit.libreoffice.org/11167
Reviewed-by: Zolnai Tamás <tamas.zolnai@collabora.com>
Tested-by: Zolnai Tamás <tamas.zolnai@collabora.com>