feat: notifications and sound alerts on AC hotplug and low battery

[mmstick]

My system powered off without any warning, so I've implemented the expected warnings and sound alerts here.

• Plays sound when entering a low battery threshold (<20%)
• Plays critical sound alert repeatedly when entering the critical battery threshold (<10%)
  • Unless or until the system's AC adapter is plugged in
• Plays sound when plugged and unplugged
  • If battery is critical, a critical sound alert begins
  • Unplugging while critical plays power-unplug-battery-low from the Pop sound theme (if the Pop sound theme is installed)
• Displays notifications for critical and low battery alerts

We should also think about automatically shutting down when the battery reaches a certain threshold to get ahead of any potential file system corruption or data loss.

I've noticed that zbus is behaving very unreliably with AC hotplug events in upower. Messages are received very sluggishly, and often dropped altogether. Seems to also happen in the example of our upower-dbus crate. Listening to /var/run/acpid.socket for AC adapter plug events was more reliable.

Partially improves #18