using a single undo document from ScMovingAverageDialog
rather than write tons of formula-cells to the document
one by one thus creating that many number of undo docs
unnecessarily.

Change-Id: I2528e0ab47f83e0c5ea40c73d00db5af14f656e0
Reviewed-on: https://gerrit.libreoffice.org/71823
Tested-by: Jenkins
Reviewed-by: Eike Rathke <erack@redhat.com>