add peripheral battery levels for mice and keyboards

[tqwewe]

Adds peripheral battery levels to the system info. An indicator is also available under Peripherals.

When the device is plugged in, disconnected, or battery level changes, the UI should reflect this.

The current implementation unfortunately makes ashell instantiate UPowerService twice, which isn't ideal for performance, but its working for now.

Did we want peripherals to be a default in the indicators?

[MalpenZibo]

I prefer to work on the Bluetooth settings module to make this happen.

Currently, the Bluetooth settings module should already display the battery level, but you need to open the settings menu first and then the Bluetooth menu. I think the whole point is to have the indicator on the main bar.

I had to think about it

[tqwewe]

Many peripherals don't depend on bluetooth. For example, my mouse has a small usb receiver I plug in, and works just fine wirelessly with Bluetooth disabled