Compute the number of rows/columns to insert based on selected cells.

Change-Id: I489bca715dcf31d191f9a875ac5d59a6140a14d7
Reviewed-on: https://gerrit.libreoffice.org/15741Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>