Inhibitive submasters

I've finally gotten around to solving this properly... I've given it some more thought (and have been reading up on some theory) and have reached the conclusion that the most logical and flexible way to implement this is with channelgroups.
With this patch, a slider can act as a submaster to a channelgroup. This is much more flexible than the original idea of using Function::adjustIntensity(): this way any channel can be submastered. This comes at the cost of slightly higher complexity. However, I managed to implement this without touching UniverseArray :)