Ducker support

[ndim]

Since a friend of mine with a Mac has recently acquired a Notepad-12FX, I have been able to generate a bit of protocol documentation for the ducker from observing the USB communication between the Mac and the Notepad-12FX.

To test all of that information without the complexity of going through D-Bus services, through the whole Python stack etc., I have written a small command line interface in C called scnp-cli, and that repo is also where I have documented the USB protocol used by the Notepad mixers, at least as far as I understand it: https://github.com/ndim/scnp-cli/blob/main/doc/soundcraft-notepad-usb-protocol.md

I plan to write proper soundcraft-utils code (especially a GUI) for the ducker with that information once I have finished #22.

[ndim]

A few thoughts about a good GUI for the ducker.

The ducker in the Notepad depends on the audio routing part of the GUI, as that determines which audio channels are actually routed into the main chip so that the main chip can use them to determine whether to duck the USB playback 1+2 stereo signal.

So the audio routing settings should be at least visible when configuring the ducker, and while they are visible, proably changeable as well.

So having the audio routing in the left part of the GUI window with the ducker controls in the right part (greyed out when disabled) could achieve all that quite nicely.

Hiding the audio routing in a tab which is not visible while the ducker tab is visible is not a very friendly user interface IMHO.

[lack]

I think that makes sense for a UI. The existing window is so small we've got a lot of room to add the ducker with it.

[ndim]

Also, allow more than one device. Stack GUI for multiple devices vertically perhaps?