Allow Homey 2.0.0 to expose a Mill heater as a triggerable device

[pardeike]

I would love to sync other devices to my Mill heater in HomeKit. Right now, the only way is to create a virtual device in Homey 2.0 and keep it in sync with a flow and then use homekit to react on changes on that virtual device.

It would be nice to expose the heater (and maybe it’s measured temperature too) as properties that carry over to the homekit world.

[pardeike]

I have played with various settings in my local fork and found that simply adding an "onoff" property will make the new Homey 2.0.0 firmware expose this nicely to HomeKit. For good measures, I also added the "thermostat_mode" property so the mobile interface shows orange/black for when the Mill is heating or not.

I then added

this.setCapabilityValue('onoff', room.isHeating),
this.setCapabilityValue('thermostat_mode', room.isHeating ? 'heat' : 'off')

to async refreshMillService() {} and removed-added my Mill and everything works fine.

It would be nice to incorporate those changes. You already have a "mill_onoff", why not change it to "onoff" or is there any other purpose of having a non-standard property name?