Doc-fr: updates LM, NR and Usage
Increase tagline spacing (Issue 3214)
Adds bar line section to LM (Issue 3408)
Issue 3468: lilypond-book and spaces in application name
Issue 3493: Let Skyline's copy constructor use generated copy constructor

Originally connected with issue 3490, now separate.
LSR updates
Merge branch 'master' into translation
Issue 3498: fatal error with toplevel-markup-identifier

The reported problem

    mrkp = \markup { "x" }

    \mrkp

triggers an error because of markup requiring additional lookahead in
assignments.  This is fixed by sacrificing string addition, an arcane
and undocumented feature that has probably not been used in the last
decade or so.
Issue 3491: Add \displayScheme command.
Doc: Describe \displayScheme.
Issue 3499: Remove 'thickness from LedgerLineSpanner interface.
Use the python found by ./configure to build the website.

This fixes issue 3494:
make website fails on platforms where python3 is the default.
trill-pitch-engraver: acknowledge stems and flags; issue 3465
changes: mention that decimal numbers can be written without #

(change introduced by commit 680df85187d0f38)