FR: differentiate on-off icons in core Lovelace

[Mariusthvdb]

Please consider adding a core option for differentiating between on and off icons.

Given the multitude in the mdi catalogue with the suffix -off, it would be great to be able to use these in the core entity panels in the icon fields.

We could dispense with the customize settings on icon, and the many integrations that don't support icon templates would benefit.

Personally I use custom-ui for setting icon template in customize. It makes for a very meaningful Frontend to be able to directly see a device is offline or not, eg using:

            if (state == 'on') return 'mdi:wifi';
            return 'mdi:wifi-off';

that could be completely dispensed with, if core Lovelace would support setting 2 icons, 1 for on (default, and a second for off) in the entity panel: