We need to disable undo handling during file import.
Same is done for other file formats (e.g. PPT, XLSX).

Change-Id: I1969c17b4a25b59b0d216ee847b3664e9c2207ca
Reviewed-on: https://gerrit.libreoffice.org/46882Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>
Tested-by: Tamás Zolnai <tamas.zolnai@collabora.com>
(cherry picked from commit 02ea9bc3)
Reviewed-on: https://gerrit.libreoffice.org/46973Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>