WIP: Introduce dedicated "Turn off keyboard backlight" action
Instead of having DimDisplay and DPMS fight over the keyboard backlight. More importantly, though, this allows letting the keyboard backlight turn off even when screen power management is inhibited. It doesn't really make sense to keep the keyboard backlight on while watching a video, does it?
- kconf update somehow to have Dimdisplay timeout copied to this new action
- test on desktop pc if the setting hides properly