Doc: Change Balloon Text example

Issue 3608

Remove redundant \balloonLengthOff command

Change paragraph to say 'does not' rather, than currently stated, as 'does'
NR: Correct explanation of ledger line override

Issue 3556

Correct the explanation of how the ledger-line-thickness
override is calculated.
Issue 3611: Parser: remove non-closed expressions from function_arglist_nonbackup_common
Revert "Issue 3205: opening bar check causes crash if \score contains the \midi block"

This reverts commit c83a3afbc51c62769cbb8963371d37b312bbcd42.

The replacement of Simple_music_iterator with Music_iterator in
lily/bar-check-iterator.cc made bar checks at the start of music
expressions inoperative altogether, even when not at a bar boundary:

{ a a { | } a a }

stopped producing a diagnostic.  Issue 3205 instead needs to be fixed
together with issue 2392 which apparently may be called "Segfault in
Midi when Simple_music_iterator is used at the beginning of an
implicitly created context".
Issue 3603: Make \tweak ... Grob equivalent to \override rather than \once \override

This simplifies several uses of tweak for both tweak and override.
When desired, one can use \once \tweak in order to get either tweak or
override-once semantics: in the case of the tweak semantics, \once
will just ignore the contained expression.
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.