[LyX/master] Math.lyx: merged changes from Phil Netro.

[Pavel Sanda]

commit 22a0ea85d0de39f80a94929768c11d31fc296590
Author: Pavel Sanda <sanda at lyx.org>
Date:   Sun Feb 7 21:40:29 2021 +0100

    Math.lyx: merged changes from Phil Netro.
    
    Most changes accepted, some remain visible in CT, because they were real bugs
    (etc. shortcuts, or mathed commands) which might be broken in other language
    variants.
    
    We will accept them for 2.4 in the final review.

 lib/doc/Math.lyx | 1545 +++++++++++++++++++++++++++++++++---------------------
 1 files changed, 955 insertions(+), 590 deletions(-)