Consider if `channelProps` behaviour on `ChannelsEnabled` should be further restricted

[seankmartin]

Since the channelProps don't have anything to watch if they have been changed, only trigger callbacks etc. in setChannelProps I think it could be a bit confusing if you directly modify channelProps on something which implements the ChannelsEnabled interface.

For example, if you call volumeLayer.channelProps![0].visible = true then this is allowed, but won't have any effect as it misses all the logic that happens in setChannelProps. Perhaps this could be improved to indicate that setChannelProps is not just allowed, but the only supported way to change channelProps on a ChannelsEnabled

Originally posted by @seankmartin in #564 (comment)