Issue 630: non-synchronized grace note makes voiceOne go stems down

Issue 630: non-synchronized grace note makes voiceOne go stems down


This is a complex issue not easy to tackle.  Part 1 of this issue
stops the end of a grace section from popping property settings it has
not pushed itself, either by finding the original push below the
actual top of the property stack (when something got pushed during
grace time) or, when not finding it, doing nothing at all.


Issue 630/1: Implement Grob_property_info::matched_pop

This can be passed a token returned from Grob_property_info::push and
will then only revert the matching push or nothing at all.  This is used
to limit the fallout from internal property reversions that might not
match the original override because of an intervening property push/pop.


Issue 630/2: Let grace_engraver only pop grob properties it pushed itself


Part 2 of this issue is concerned with the start of the grace section.
It tries figuring out whether any property setting commands adjacent
to \grace came immediately before or behind it and execute them in the
proper order.  This can only work by letting the Grace_iterator
determine the timing.  It communicates the timing via a GraceChange
event to the corresponding Grace_engraver.  As there are several ways
in which the connection between iterator and engraver may be severed,
fallbacks are implemented.  When they are being used, property setting
commands at the start of the \grace group may be executed before the
grace group sets its grace-specific properties.


Issue 630/3: Let Grace_iterator produce a GraceChange event


Issue 630/4: Let Grace_engraver react mostly to GraceChange events

When GraceChange events are not available, this reverts to grace processing
at initialization or at the engraver's process_music call.

[dak, 2014-10-02 22:09:37]

Reinstate acciaccatura in tab example removed during issue 2480

[dak, 2014-10-02 22:35:23]

Move override in flags-straight-stockhausen-boulez.ly