Selectable midi channel input to arranger mode

[buck5125]

Being able to set a keyboard split point is perfect for a single manual keyboard, but on a keyboard with upper and lower manuals like the EL-90 and similar midi capable Yamahas they transmit separate midi channels for each keyboard and pedals. The arranger currently responds to notes on either keyboard, so to avoid melody notes confusing the chord, the ability to selectively assign midi channel here (including default ALL) would be fantastic.