PowerPoint renders these as bent connectors, not as arrow shapes.

Also add a bit of vertical spacing between the nodes, otherwise the
connectors have no way to be visible. Their position is still incorrect,
though.

Change-Id: I995930c0bbc1bdb1014face2490be392571548a2
Reviewed-on: https://gerrit.libreoffice.org/66627Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins