Move the line spacing member to the TextParagraphProperties class
which is used to do the inheritance from master / layout slides.

Change-Id: I0bf92420963163eae78e06ebc7fdfa1f2c72fdf8
Reviewed-on: https://gerrit.libreoffice.org/49692Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>