applets/brightness: show label for keyboard brightness level

Currently keyboard backlight intensity is not shown in a label because in !108 (comment 65417) it was said that showing a three step keyboard brightness as 33/67% is bad.

Instead of hiding the information, show informative strings (off/low/medium/high) if the backlight has few levels, and a percentage if it has at least 5 (e.g. my keyboard has 50, where a percentage is certainly meaningful).

This is both more informative and adds visual consistency with other items in the brightness and battery applets where we show the current value to the right of the heading.