power: Don't warn more than once per warning level for devices

A lot of wireless input devices, such as Logitech mice and touchpads, or Bluetooth LE input devices, will disconnect and reconnect frequently from the computer when unused.

This causes a problem when the battery on the device is low because a new notification will be generated each time the device reconnects, as if it was the first time it connected.

Saving the last warning-level for every external peripheral ensures that we only emit one low battery notification for each warning-level, when the threshold is crossed, rather than at every reconnect.

The last warning-level is not stored, so a new session, or a reboot will cause devices to warn again once.

Helps: #108 (closed)