Issue 3581: New data types for adjusting MIDI control functions

Signed-off-by: David Kastrup <dak@gnu.org>
Issue 3581: Allow overriding Translator::{connect_to,disconnect_from}_context

This is needed to make it easier to implement a custom translator that will
listen to SetProperty events.

Signed-off-by: David Kastrup <dak@gnu.org>
Issue 3581: Add support for changing MIDI balance, pan position, reverb and chorus levels

Signed-off-by: David Kastrup <dak@gnu.org>
Fix the default value description for midiChannelMapping

The current default value is "staff", not "instrument" (noted during
the review of patches related to issue 3581).

Signed-off-by: David Kastrup <dak@gnu.org>
Remove definition of the "instrumentName" property from Score_performer

This setting has no effect; furthermore, according to the discussion on
issue 3581 and the related patch review comments, no default settings for
MIDI controls should ever be enforced if they are not set explicitly.

Signed-off-by: David Kastrup <dak@gnu.org>
Merge branch 'issue3581' into HEAD

Done as a single merge commit as I don't know how well intermediate
stages might compile.
parser.yy: remove unused CHORDMODIFIER_PITCH and CHORDMODIFIERS
stencil.scm: \hspace is not special in fill-line;issue 3504
key-signature: restore function for LilyJAZZ.ily; issue 3606

key-signature-interface::alteration-position
Issue 3618/1: Eliminate EXPECT_PITCH token in parser
Issue 3618/2: Parser: replace is_duration with more versatile make_duration function
Issue 3618/3: Eliminate EXPECT_DURATION token in parser
Issue 3618/4: big rewrite of optional argument scanning.  See comment inside.
Merge branch 'issue3618' (early part) into staging

Not sure whether every state in between will perfectly bisect, so
using a merge commit.
Parser: remove unused operator priorities
EG: Some updates on music functions
Release: bump VERSION_DEVEL.
PO: update template.
Release: update news.
Midi/GUB bugfix

lround is not supported in older C/C++ libraries, so use my_round instead.