add support for ac power supply

It would be really useful for me to see if the AC power supply is plugged and working. What do you think about adding support for it?

My `/sys/class/power_supply/AC/uevent`:
```
POWER_SUPPLY_NAME=AC
POWER_SUPPLY_ONLINE=1
```

Start by tracing how existing power-supply data is discovered and displayed, using /sys/class/power_supply/AC/uevent as the reported Linux entry point. Add support for the POWER_SUPPLY_ONLINE value and verify that the status bar reflects whether AC power is connected and working.

Written by the indexing model from the issue text.