[CM] audio plots

[Heinrich Taube]

svn trunk now has some big improvements to plotting:

you can now generate audio from ANY plot. just add points, open a midi  
out port, then choose Audio>Play Plot...  from the plot window's audio  
menu.  You can control how various point values are mapped to sound  
using the Play Plot dialog.

support for piano roll plotting ( :style :hbox)   has now been improved

the implementation of layers has been generalized, it will be possible  
soon to 'pivot' layers to show any pair of fields in mutidimensional  
points .

the Help>Examples>Plotting  example file now includes michael  
klingbeil's old arpa harmonics example working.

the XML representation for plots had changed a bit -- if you need to  
load an older .xlm file ill tell you what to do to get it to load (its  
easy to fix)