This is specifically for the benefit of DOCX import, but it
also makes sense in general. If a SwXCell is given char/para
properties, then apply those properties (without overwriting)
to the cell's contents.

This allows ANY paragraph or character properties that are applied
to a table style to become the "default" for the table.

This fixes a number of things:
-remove one-off hack to get PROP_PARA_LINE_SPACING to work.
-works for all character and paragraph properties (except those
shared with tables like borders).
-works in multi-paragraph cells. Previously those could return
AMBIGUOUS state, in which case the style wasn't applied at all.

Change-Id: Ia98c129879575c1aa8ca1fe2a64f4991c0a264e8
Reviewed-on: https://gerrit.libreoffice.org/54511Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Justin Luth <justin_luth@sil.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>