Implement keyboard navigation for battery monitor

When opening the popup the brightness slider has focus so you can immediately adjust brightness
using left/right arrow keys. Tab moves you to the keyboard brightness slider, then to the PM
checkbox which you can toggle using Return/Space, then to the battery list through which you
navigate using arrow up/down and expand/collapse using Return/Space/Arrow left/right.

FIXED-IN: 4.10.95
BUG: 228486