Handle `kbdillumtoggle` (toggle the keyboard backlight)

My laptop (and I suspect others too) only has a single Fn key for the keyboard backlight. Currently, only the up and down keys are are recognized. The kbdillumtoggle (XF86XK_KbdLightOnOff) could be configured to cycle through the available brightness levels.

I have a patch that does that, but I cannot open merge request due to insufficient permissions in this site