rhbz#1065629: RTF import: re-implement nested tables

The previous fix for this bug only fixed a symptom, this a fix for the
real problem; with the real problem fixed the nCellEnds is unnecessary.

Given that top-level table properties may be put either before or after the
table cells, the only way that works to import tables is to buffer a whole
top-level table row, but currently the buffer is replayed already at the
end of a nested table row.

Fortunately the RTF spec guarantees that \nesttableprops must occur
after the nested table cells of the nested row, so it should be
sufficient to remember the cell properties for the current nested table
row only, in addition to the cell properties for the top-level table row.

With this change, skipping a \nesttableprops destination when there is
a table style turns out to mangle ooo98040-1.rtf badly, so stop doing
that workaround.

RTFDocumentImpl::popState() was copying various buffers up the state
stack which is a clear indication that these shouldn't be members of
RTFParserState in the first place, move them to RTFDocumentImpl.

(cherry picked from commit c8f83ce1dbf5544f6aaa91775db6820a69c3c061)

Conflicts:
	writerfilter/source/rtftok/rtfdocumentimpl.cxx

Change-Id: Ic2d8f7b3e00844b224d61605b405ca651239e5f7
Reviewed-on: https://gerrit.libreoffice.org/8221
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>