Proposal: Improved 3-channel interface (implementation attached)

[fwiffo]

Filing this to track the work I've been doing on some improvements for three channel lights.

• Reorganizes channels on 3-channel D4K to be consistent with the sequence of aux LEDs (channel 1 then 1+2, 2, 2+3, 3, 3+1, then 1+2+3). After that is HSV mode, then auto-tint mode. RGB aux channels follow that.
• Adds a second set of channel args which can be adjusted with 4H (overriding momentary turbo channels that require secondary args).
• The secondary args control saturation in HSV mode, so all possible blends of the three channels are accessible from that mode in a reasonably intuitive interface.

It also adds a couple new strobe modes.

• Hue cycling mode - you can control the speed (and direction) over a range of speeds from 1 cycle per 3 minutes to 10 cycles per second. Brightness and saturation can still be adjusted in this mode.
• A blinking sequence zig-zagging around the three channels.

This implementation merges in the channel_uses_aux() PR. #33

I will attach a pull request for the implementation.