bnc#657899: Paragraph vertical spacing differs from PowerPoint

We need to set the "paragraph summation" flag also for .pptx import. (As is
already done for .ppt import.)

This is what in the Options (Preferences) UI is called "Add spacing between
paragraphs and tables (in current document)", on the LibreOffice
Impress:General page. This is a setting that when set becomes local to the
document. I actually don't really understand why this needs to be a
user-visible option, wouldn't it be enough for it to be an internal flag that
is set for documents imported from .ppt and .pptx, and is clear otherwise?

Change-Id: I525178957276e7d587b6ac4be8ae7919fc245a27
(cherry picked from commit 96d210bf9ca0a99fc8256d6e0a4952207cce69e0)
Signed-off-by: Andras Timar <andras.timar@collabora.com>