Updating translators

[Erik Sandberg]

Hi,

The attached patch makes it possible for translators to listen to stream 
events directly.
1. Stream events are now probs, with 'class as the only immutable property.
2. Music::send_to_context now sends two stream events for each music event: 
One old-style OldMusicEvent, and one MusicEvent. The event's mutable property 
list is copied directly from the music event.
3. Some mechanisms have been added to Translator, that make it possible to 
register methods to listen directly for event-classes.
4. As a first demo, arpeggio-engraver has been rewritten to use these 
mechanisms.

The patch is attached; it compiles regression tests nicely.

Problems:
- A side-effect of (2) is that each event is sent twice; therefore, I have 
temporarily disabled a warning message.
- I think there are potential GC problems: Most translators don't seem to 
protect the try_music parameter; they trust that someone else protects it for 
them. This assumption is sufficient for Music (there's always someone higher 
up who protects incoming Music), but I don't think you can assume the same 
for Stream_events. Some possible solutions:
  A. Rewrite all translators to protect incoming events appropriately. This is 
the most efficient solution, but it requires more work when creating new 
translators. There are currently about 50 translators for which new 
derived_mark methods will have to be created.
  B. Create a dummy listener that protects all incoming events, and unprotects 
them at the end of each moment. The easiest way is probably to let 
Score-level translators take care of this. This is the easiest and safest 
solution, but also the most inefficient one.
  C. Create some macro trickery in translators, so that incoming events are 
remembered and protected automatically between try_music and 
stop_translation_timestep.
  D. Add an 'event property to Music, so that sent events are protected by 
their music origins. This is the easiest solution, but I'm nervous about 
memory errors that can happen when a different front-end is used (e.g., when 
loading music streams directly from a file)

-- 
Erik

es-060707.diff
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