Finally, we have support for sending out Battery information.

Summary

The core idea is as follows:

1. When a Link loads the BatteryPlugin, we query Solid for a list of batteries.
   1. If the list is empty, we print a warning message and return quickly
   2. Otherwise, we connect *two signals* to every object in that list
2. We send out a single new NetworkPacket as soon as we've processed that list
3. When either of those two signals emits, we send another new NetworkPacket

Multi-battery Support

BUG: 357193

To handle devices with multiple batteries (requested in that bug), we average
together the battery percentages. This also includes a new field in the packet for
'number of batteries' called batteryQuantity. For backwards compatibility, we can
assume it has a default value of one.

This should ensure we support

• devices with no batteries at all (like many desktop machines)
• devices with hot-pluggable batteries (like those laptops with detachable screens)

Concerns

Note that the implementation isn't perfect.
We'll need some new localizable text to make it clear that we now support sending
battery status information.

Then there's a rather significant question: maybe we should have two battery plugins
on each client, like we do for the findmyphone/findthisdevice plugins?

Test Plan

We need to ensure that other clients (including those using the Android codebase)
will respond correctly. The main things to look at are

1. are these new packets sent when the plugin is enabled, and not sent when it's disabled?
2. is the charge percentage accurate?
3. is the charge state (charging, discharging, or full) accurate?

and

4. do we see the correct number of warnings for low-battery?