Make LEDOutput usage simpler by removing "standby PWM" memory

[seanlano]

Currently there is code to have the LEDOutput class remember the previous power level when turning off. It also means the class must both be given a power level separately when turning on. This is unnecessary and confusing.

Instead, the LEDOutput class should:

• Remove setPowerOn and setPowerOff methods, and require the use of one of the setDim methods
• Ensure in all cases that the setDim methods also turn on the power (this should already be the case)
• Remove __state_pwm_standby and other now unnecessary variables