show OSD when keyboard backlight changed through hardware

Natalie Clarius requested to merge work/natalie/kbd-backlight-hw-changed into master

Show the brightness OSD when upower signals that the keyboard backlight changed through hardware, eg a firmware-handled shortcut (often Fn+Space to cycle brightness levels), for which so far no OSD was shown.

This is a blind fix as I don't have any device with such a shortcut; can someone who does please test this? @nicolasfella

BUG: 406790


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