keyboard backlight: support multiple led devices

Closes #237. On machines with multiple keyboard backlight led devices, upower up until now would only consider one (semi-random) led and ignore the rest. This has the effect that eg user controllable keyboard backlight control in KDE Plasma only changes the light of one key on the keyboard while the other keys can not be controlled. This commit fixes this by collecting all /sys/class/leds/*kbd_backlight* entries instead of just the first one found, and appyling the read and write operations to all of them.