All performers converted.

[Erik Sandberg]

Hi,

I have finished converting all performers, as a warming-up round (it made me 
realise which obstacles I will encounter when converting engravers).

I also encountered and worked around a problem with engravers: The 'cause 
property of a grob is currently set to a music object sometimes. Eventually 
I'll change this to stream-event everywhere, but until then I've implemented 
a hybrid solution: Music causes are stored as stream-events internally, and 
Grob_info::music_cause converts that event back to Music. (to make things 
easy, I simply added a music-cause property to stream-events, which links 
back to the originating music. That property will be removed later.)

New methods for reading event_cause have been added as well.

I also removed the unused BusyPlayingEvent.

-- 
Erik

es-060717.diff
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