Fix t/40-system.t after ::System->loadavg rename to gen_loadavg
t/40-system.t: also test fetch_battery
Rename AngelPS1::System->fetch_battery to gen_fetch_battery

... because this method has an indirection level: it returns
a sub that will fetch the value, not fetch it directly.
t/53-Battery.t: reverse testing order

Now, from full charge to discharged.
BatteryGauge: use semantic names for color constants
BatteryGauge: color bold Blue if charging