[CM] csound port improvements

[Heinrich Taube]

rev 1576 of Grace adds Csound support for scores and non-realtime  
audio , Common Music Loop macro added to Scheme and new (incomplete)  
documentation for the Port Menu.

documentation on the new Cound port functionality including snapshots  
of its dialogs is here:

http://pinhead.music.uiuc.edu/~hkt/grace/doc/ports.html#csound-port

and explanation of how ports support rt and score work is at the top  
of the file.
over the next week or so Ill add the same sort of score support to the  
Midi port so its menus will change a bit.