Add percentage display next to the brightness and keyboard backlight sliders in KCM

Currently if you were to open System Settings and go to the
Power Management -> Energy Saving KCM, you will see the "Screen Brightness"
and "Keyboard Backlight" sliders, but there will be no way to know the exact
number you selected until you apply the change and look at the "Energy Saving"
applet. This commit changes that by adding percentage labels to the right of
both sliders.

BUG: 440314
FIXED-IN: 5.25