fdo#75757: remove inheritance to std::vector

from ParagraphObj, by using boost::ptr_vector.

This is also a supposed fix for ParagraphObj::operator=(),
because ParagraphObj::ImplClear() now makes the vector empty
while its original code kept deleted pointers.

Change-Id: I7710eb33fc05bed55474ff00baaeb8e19cd79f3d
Reviewed-on: https://gerrit.libreoffice.org/11379
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>