Move Tweak_engraver to Score level

Move Tweak_engraver to Score level

This makes it possible to tweak items announced at Score level, like
MetronomeMark and RehearsalMark.

The advantage is that tweaks will be applied once regardless of the
context structure (the Score context should exist only once).

Due to the hierarchical nature of acknowledgers, acknowledgers in
lower contexts will now get to see the grobs before tweaks have been
applied.  However, grobs are still unfinished (except for type,
properties initialized via context properties and cause) at the time
they are announced, with other details only getting filled in by the
engraver after announcement, so the potential for trouble seems low.
Acknowledgers should usually just register a grob (or write grob data)
with any actual reading of grob data occurring at the end of the
timestep instead.

[janek, 2013-04-04 10:19:14]

LGTM. I learned something, thanks :)
Janek

[dak, 2013-04-04 10:30:39]

Regarding the description: "Acknowledgers should usually just register a grob
(or write grob data) with any actual reading of grob data occurring at the end
of the timestep instead."

Strictly speaking, this principle has been violated already by allowing
subproperty tweaks, as tweaking a subproperty requires reading the main property
first.

Moving the Tweak_engraver to Score level actually makes it more likely that
other acknowledgers potentially writing a main property will have acted before a
subproperty tweak is attempted.

In this respect, this patch should decrease the likelihood of surprises (though
not ruling them out altogether).

[janek, 2013-04-04 10:40:38]

On Thu, Apr 4, 2013 at 12:30 PM,  <dak@gnu.org> wrote:
> Regarding the description: "Acknowledgers should usually just register a
> grob (or write grob data) with any actual reading of grob data occurring
> at the end of the timestep instead."
>
> Strictly speaking, this principle has been violated already by allowing
> subproperty tweaks, as tweaking a subproperty requires reading the main
> property first.
>
> Moving the Tweak_engraver to Score level actually makes it more likely
> that other acknowledgers potentially writing a main property will have
> acted before a subproperty tweak is attempted.
>
> In this respect, this patch should decrease the likelihood of surprises
> (though not ruling them out altogether).

That's good! :)