fdo#79319: writerfilter: RTF import: support horizontal rule

There are special properties to create a "horizontal rule" shape
that apparently set some specific defaults; this prevents the
shape being imported as a big fat rectangle over the document.

Change-Id: I402376d7306e870ad895beaa657750cbf3290d98
(cherry picked from commit 589ca2a5e88a976bb10e60fcb1e3e75f4aa2504e)
Reviewed-on: https://gerrit.libreoffice.org/10388
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>