Issue 3296: Move Tweak_engraver to Score level

Issue 3296: 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.