[LyX/master] Simplify code to generate only one paragraph at a time.

[Thibaut Cuvelier]

commit 87dae26e4a8146c7e9b450dfff4d8711deeed933
Author: Thibaut Cuvelier <tcuvelier at lyx.org>
Date:   Sun Aug 16 00:59:43 2020 +0200

    Simplify code to generate only one paragraph at a time.

 autotests/export/docbook/basic.xml |  355 +++++++---------
 autotests/export/docbook/easy.lyx  |  253 +++++++++++
 autotests/export/docbook/easy.xml  |   68 +++
 autotests/export/docbook/lists.lyx |   18 +-
 autotests/export/docbook/lists.xml |   48 ++-
 lib/layouts/stdlayouts.inc         |   10 +-
 lib/layouts/stdlists.inc           |    4 +-
 lib/layouts/stdstarsections.inc    |    4 +-
 lib/layouts/stdstruct.inc          |    5 +-
 src/Paragraph.cpp                  |    9 +-
 src/ParagraphList.h                |   11 +
 src/insets/InsetBibtex.cpp         |    3 +-
 src/insets/InsetERT.cpp            |    3 +-
 src/insets/InsetNewline.cpp        |   21 +-
 src/mathed/InsetMathHull.cpp       |    3 +-
 src/output_docbook.cpp             |  811 ++++++++++++++----------------------
 src/xml.cpp                        |   45 +--
 17 files changed, 908 insertions(+), 763 deletions(-)