expand mouse_support to three values rather than boolean

expand mouse_support to three values rather than boolean

On Windows only:
   0 = turn off mouse_support
   1 = turn on mouse_support and turn off QuickEdit mode
   2 = turn on mouse_support and leave QuickEdit mode untouched

More generally, but not implemented anywhere:
   0 = turn off mouse_support
   1 = turn on mouse_support and make supporting O/S adjustments
       (O/S adjustments not implented beyond Windows as yet)
   2 = turn on mouse_support and do not make OS adjustments
       (unimplemented as yet so behaves as 1)