Moots: add a slewing option for each control unit

[cosinekitty]

This idea comes from a feature request by Artem Leonov and subsequent conversation with David Benham.

When a control unit is toggled on or off (either by push-button or gate signal), it responds immediately. This is desirable for control voltages, but can cause clicks when used for audio signals. In the latter case, it would be nice to have an option to cause a linear fade out (slew) before disconnecting the cable, or to fade in from 0V immediately after connecting a cable.

The VCV sequential switch modules, 1-to-4 and 4-to-1, slew for 1/400 of a second using a linear ramp, so I will do the same when slewing is enabled.

The Moots front panel design will not change. I will add context menu items to toggle the slew option for each of the 5 control units.

Slewing will be disabled by default.