Use 3 symbolic colours to paint the levels in the battery bars, with a
red "error" colour used for the lowest level of battery, an orange
"warning" colour for the pre-error level, and a green "success" colour
used for levels above that.
There's no yellow intermediate colour as this is usually too anxiety
inducing and there's no real need to press users into a "warning"
behaviour when the level will still be comfortable for a long enough
time.
Closes: #725
