Improve fan speed controls

[jasongin]

This change makes the fan speed controls work better in a few ways:

• The fan slider snaps to 20% increments, corresponding to the 5 fan speeds supported by the Kumo API.
• The previous mapping from slider percent to Kumo API values would fail for part of the slider range due to the valid values being 0,1,2,3,5,6 (no 4). This is resolved.
• When turning on the fan by setting it to active ("Hey Siri, turn on the Living Room AC Fan"), it now turns on to max speed instead of min speed. (Min speed is so slow and quiet that I assume nobody wants that as the default.)
• When turning off heating/cooling, the system mode is set to fan if a specific fan speed had been set; otherwise the system power is turned off. Other interactions between fan speed, mode, and "active" states are improved as well.
• With the above improvements to handling active states, the "Power" switch seemed redundant, so I removed it. Please review this choice, as I'm not sure if some people might have been using this switch in some way I'm not aware of.

I tested these changes with both direct and cloud APIs. Some additional verification from another user/developer might be helpful, since there are some complex interactions here.

All of the changes are applied only to the "ductless" device class. Some of the improvements could be ported to the "simple" or generic device classes; please advise if you think this is necessary.

[jasongin]

I have another big change coming up that builds on this one, that supports disabling some functions. (I only care about air-conditioning and fans so I don't want to see a mode selector that offers heating.) But I'm going to bring that later as a separate PR, so they will each be easier to review.

[Jason-Morcos]

@jasongin With these improved fan selection options, how do you specify an "auto" fan speed?

[jasongin]

With these improved fan selection options, how do you specify an "auto" fan speed?

If the AC is on and the fan speed control is set to off, then fan runs in "auto" mode. If the AC is off and the fan speed is set to off, then the system is fully off.