setPowerState should take a duration

[lightbow]

You can change a light's color over a specified period of time, but the power state payload does not take a duration. It would be great to have a duration parameter for setPowerState just like for setColor.

• (void)setColor:(LFXHSBKColor *)color overDuration:(NSTimeInterval)duration;
• (void)setPowerState:(LFXPowerState)powerState;