[LyX/2.3.x] Add support for the doublestroke package

[Enrico Forestieri]

commit 10e1f9088301bf50c34744803bf50e24745136e2
Author: Enrico Forestieri <forenr at lyx.org>
Date:   Fri Jun 12 23:58:32 2020 +0200

    Add support for the doublestroke package
    
    This commit adds the mathed command \mathds that selects a
    font more appropriate than \mathbb for typesetting the
    mathematical symbols for the natural (N), whole numbers (Z),
    rational numbers (Q), real numbers (R), complex numbers (C),
    and some others.
    
    As in the \mathbb case, only capital letters are supported,
    but in addition one can also typeset a symbol often used for
    representing the indicator function (\mathds{1}) and the
    letters a, h, k.
    
    Fixes bug #11887.

 lib/Makefile.am                     |    1 +
 lib/doc/LaTeXConfig.lyx             |   63 +
 lib/fonts/dsrom10.sfd               | 2355 +++++++++++++++++++++++++++++++++++
 lib/fonts/dsrom10.ttf               |  Bin 0 -> 9944 bytes
 lib/images/math/mathds.svgz         |  Bin 0 -> 751 bytes
 lib/symbols                         |    1 +
 lib/ui/stdtoolbars.inc              |    1 +
 src/Font.cpp                        |    4 +-
 src/FontEnums.h                     |    2 +
 src/FontInfo.cpp                    |    3 +-
 src/LaTeXFeatures.cpp               |    3 +-
 src/frontends/qt4/GuiFontLoader.cpp |    5 +-
 src/mathed/InsetMathFont.cpp        |   30 +-
 src/mathed/MathFactory.cpp          |    2 +
 src/mathed/MathSupport.cpp          |    2 +
 status.23x                          |    2 +
 16 files changed, 2453 insertions(+), 21 deletions(-)