[CM] plot hooks

[Heinrich Taube]

svn trunk now has support for 'plot hooks' --  scheme functions you  
associate with a plot window. if a hook is registered it is called  
when you do control-option-mouseclick in the window. it should return  
a list of point(s) to add to the plot given that mouse postion or ()  
if nothing should be added. see the cool fm example in the  
Examples>Plotting  file. documentation in res/doc/cm.html#plotting