Mute channels for NCSF

[GuiMiTomo]

A move of this CyberBotX/SDATStuff#6 (comment) because I feel I posted that in the wrong place.

[CyberBotX]

It depends on if you mean a mixer channel or an output channel. The latter can already be done. The former is more of a mess to handle at all because SSEQs are played with dynamic channel allocation, meaning that a new note will play on the first available channel from the list of channels that are valid for the type of note being played. This is also different from the tracks within an SSEQ. I don't know if I plan to add support for muting mixer channels, but I'd be willing to consider it.