[Feature] Auto dimming of backlight after period of inactivity or low battery

[kiteretro]

As title suggests, with the following notes:

• Last input timer, when exceeded the backlight will dim to lowest until a button input is given
• Some kind of override to disable this when required (serial command)
• Gentle backlight adjust (ramp the PWM value rather than abruptly change)
• On low battery, set brightness down to save power

[kiteretro]

Implemented auto dim.

Did NOT implement:

• Ramped adjustment (smooth change)
• Serial command to disable