Add support for saving/restoring keyboard backlights

Piggy-backing on the display backlight code, this saves and restores
keyboard backlights on supported devices.

The detection code matches that of UPower:
http://cgit.freedesktop.org/upower/tree/src/up-kbd-backlight.c#n173

https://bugs.freedesktop.org/show_bug.cgi?id=70367

[tomegun: also work for devices named "{smc,samsung,asus}::kbd_backlight"]