In a previos move BackingWindow was moved, but startcenter.ui was left
behind by accident.

Change-Id: I7d9669df9b2b65311e119640ba2a7a331b6336e9
Reviewed-on: https://gerrit.libreoffice.org/5316Reviewed-by: Jan Holesovsky <kendy@suse.cz>
Tested-by: Jan Holesovsky <kendy@suse.cz>