Implement short press of power button

Feature: https://source.puri.sm/Librem5/use-cases/issues/47

The power button should have two modes: long press to turn the phone on/off and short press to turn the screen on/off. The long press functionality has already been implemented for turning the devkit off (doesn't work for turning devkit on - see https://source.puri.sm/Librem5/linux-next/issues/32) but the short press functionality needs to be implemented.