Use the same solution as for images in this commit:
9fee132c

Place the inserted text shape to the center of the slide
for LOK, because we don't get correct window size here.

Change-Id: Ie0c3717d22da6b6a988ab8d566bd9759e03f86c8
Reviewed-on: https://gerrit.libreoffice.org/50074Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>