fade-out leds when inactive, restore brightness based on last call to led_set()

1. Add a fade-out effect when dimming the keyboard backlight due to inactivity.
2. Restore the backlight to the last rgb value set by led_set() instead of the hsv value set by led_set_hsv(). This makes the dimming work correctly if some parts of the firmware use led_set() instead of led_set_hsv() (such as during initialization).

The firmware does not currently cache the rgb values set by led_bitmap(), so the fade-out and restored rgb color will reflect whatever color was last set with led_set(), independent of the led bitmap loaded in the keyboard. To properly fade and restore the led_bitmap we could cache the bitmap rgb at the expense of some memory.