Longer term the core Impress shape has to be improved so that it can have
text in multiple columns.

Shorter term, map text with multiple columns to table shapes, that gives
correct layout in many cases and requires changes to the import filter
only.

Change-Id: Id7942b16882ab1f083ded2f0e577e8c9e128697c
Reviewed-on: https://gerrit.libreoffice.org/60859Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins