Gray out pattern FF in the order editor

[ctrlaltmilk]

When I'm composing, I often want to leave a specific channel silent for an order. As far as I've seen, the common way in tracker music to leave a slot "empty" is to use pattern FF. I'd really appreciate the option to gray out a specific pattern, since it would provide a better indication of which channels are unused at which times. If you don't feel like mandating pattern FF as the empty pattern, it could also work to just gray out any empty patterns.