adding MIDI stop control to Aeolus.

The protocol uses one controller number. The default is #98, but you can change this in global.h. The message is accepted only on channels enabled for control in the midi matrix.

The value is interpreted as follows:

 v = 01mm0ggg 

     mm = mode. This can be:
   
        00  disabled (initial value)
        01  set off
        10  set on
        11  toggle

     ggg = group, one of the button groups as defined in the instrument
     definition. In the GUI groups start at the top, the first one (for
     division III) being group #0.

     The values of mm and ggg are stored and need not be repeated unless
     they change.

 v = 000bbbbb

     bbbbb = button index within the group.

Enjoy !

-- FA