Yet another 2 patches for music streams

[Erik Sandberg]

Hi,

I've finished 2 more patches for music streams. One of them is considerably 
larger than the other, but the two should be independent.

Pathc 1:
 - lyric-combine-music.cc, partcombine-iterator.cc: updated to use stream 
events for busy testing.
 - chord-tremolo: massively cleaned up (iterator either generates ChordEvent, 
which is handled like :, or generates span start/stop events. Engraver is 
shrinked)
 - Translator_group::try_music is now invoked through listeners. 

Patch 2:
 - output-property-*: prepare for bottomification of try_music
 - exported some more rules to scheme:
  - \once is converted to music function (we might later want to make it a 
music macro)
  - context-specification -> ly-syntax rule
  - ly-syntax rule for music functions.


Two generic utility functions have been introduced, chordify-event and 
music-is-of-type?

Todo:
- create contexts through music streams (add new event for context creation 
announcements)
- make try_music auto-descend to bottom context. Some of the current places 
where stream-events are sent should then be replaced with try_music calls.
- Make other commands (SetProperty, ChangeContext, Prepare, OneTimeStep) use 
stream events.

Questions:
- It seems that the OutputPropertySetMusic music type is unused. Should I 
remove it?
- I have not yet done anything about old-lyric-combine-music-iterator. Should 
I spend time on making it work, or can we junk oldaddlyrics for the next 
release? (there's no regression test for oldaddlyrics, btw)
- What's the Right Way to pass warnings and errors from Scheme code? I want 
some warning messages to make use of Input objects.

-- 
Erik

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