Don't unmute channels in the running instance when saving (but write them unmuted to the file)

[pulkomandy]

It is possible to save a song with muted channels. This makes no sense.

[pulkomandy]

[pulkomandy] Appears to be an item left from the 1.1 TODOlist, as this was already fixed in 1.2

There's a small bug however: when saving, the channel will be unmuted in the opened instance, and the mixer window will not uncheck the mute checkbox to show it.